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KumoCorp/kumomta
Tutorial
Tutorial
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Server Environment
System Preparation
Installing KumoMTA
Configuring KumoMTA
Starting KumoMTA
Testing KumoMTA
Checking Logs
Next Steps
User Guide
User Guide
General
General
Preface and Legal Notices
About This Manual
How to Report Bugs
How to Get Help
Credits
History
Architecture
Lua Fundamentals
Installation
Installation
Installation Overview
Server Environment
Server Hardware
Operating System
System Preparation
Security Considerations
Installing on Linux
Upgrading
Installing on Docker
Building from Source
Configuration
Configuration
Configuration Concepts
Lua Policy Helpers
Example Server Policy
Configuring Spooling
Configuring Logging
Configuring SMTP Listeners
Configuring Inbound and Relay Domains
Configuring Bounce Classification
Configuring Feedback Loop Processing
Configuring HTTP Listeners
Configuring Sending IPs
Configuring Queue Management
Configuring Queue Rollup
Configuring Traffic Shaping
Configuring DKIM Signing
Operation
Operation
Starting KumoMTA
Getting Server Status
Troubleshooting KumoMTA
Injecting Messages using SMTP
Injecting Messages using HTTP
Routing Messages Via Proxy Servers
Viewing Logs
Canceling Queued Messages
Additional Utilities
Using the kcli Command-Line Client
KumoProxy SOCKS5 Server
Policy
Policy
Checking Inbound SMTP Authentication
Delivering Messages Using SMTP Auth
Custom Destination Routing
Routing Messages via HTTP Request
Routing Messages via AMQP
Routing Messages via Kafka
Storing Secrets in Hashicorp Vault
Publishing Log Events Via Webhooks
Rewriting Remote Server Responses
Clustering
Clustering
Deployment Architecture
Aggregating Event Data
Implementing Shared Throttles
Clustered Traffic Shaping Automation
Scaling Clusters Up and Down
Deploying KumoMTA on Kubernetes
Performance
Performance
Architecture
Linux Tuning
DNS
Performance Testing
Understanding KumoMTA Message Flows
Log Hooks
Integrations
Integrations
EmailElement
Ongage
Mautic
Postmastery
Tatami Monitor
Prometheus
Grafana
Hornetsecurity Spam Filter
Rspamd Spam filter
Reference Manual
Reference Manual
Predefined Metadata
Queues
Configuration Lifecycle
SMTP Server Events
Memory Management
Template Syntax
Template Syntax
cycler
dateformat
datetimeformat
filesizeformat
joiner
normalize_smtp_response
now
pluralize
timeformat
Log Record
Access Control
kcli
kcli
kcli bounce-cancel
kcli bounce-list
kcli bounce
kcli inspect-message
kcli inspect-sched-q
kcli provider-summary
kcli queue-summary
kcli rebind
kcli set-log-filter
kcli suspend-cancel
kcli suspend-list
kcli suspend-ready-q-cancel
kcli suspend-ready-q-list
kcli suspend-ready-q
kcli suspend
kcli top
kcli trace-smtp-client
kcli trace-smtp-server
kcli xfer-cancel
kcli xfer
module: kumo
module: kumo
apply_supplemental_trace_header
available_parallelism
bump_config_epoch
configure_accounting_db_path
configure_bounce_classifier
configure_local_logs
configure_local_logs
back_pressure
compression_level
filter_event
headers
log_dir
max_file_size
max_segment_duration
meta
min_free_inodes
min_free_space
per_record
configure_log_hook
configure_redis_throttles
define_spool
define_spool
flush
kind
min_free_inodes
min_free_space
name
path
rocks_params
disconnect
eval_config_monitor_globs
generate_rfc3464_message
get_memory_hard_limit
get_memory_low_thresh
get_memory_soft_limit
glob
inject_message
invoke_get_egress_path_config
invoke_get_egress_pool
invoke_get_egress_source
invoke_get_queue_config
json_encode
json_encode_pretty
json_load
json_parse
log_debug
log_error
log_info
log_warn
make_egress_path
make_egress_path
additional_connection_limits
additional_message_rate_throttles
additional_source_selection_rates
allow_smtp_auth_plain_without_tls
banner_timeout
connect_timeout
connection_limit
consecutive_connection_failures_before_delay
data_dot_timeout
data_timeout
dispatcher_wakeup_strategy
ehlo_domain
ehlo_timeout
enable_dane
enable_mta_sts
enable_pipelining
enable_rset
enable_tls
idle_timeout
ignore_8bit_checks
low_memory_reduction_policy
mail_from_timeout
maintainer_wakeup_strategy
max_connection_rate
max_deliveries_per_connection
max_message_rate
max_ready
max_recipients_per_batch
no_memory_reduction_policy
openssl_cipher_list
openssl_cipher_suites
openssl_options
opportunistic_tls_reconnect_on_failed_handshake
prohibited_hosts
rcpt_to_timeout
reconnect_strategy
refresh_interval
refresh_strategy
remember_broken_tls
rset_timeout
rustls_cipher_suites
skip_hosts
smtp_auth_plain_password
smtp_auth_plain_username
smtp_port
source_selection_rate
starttls_timeout
suspended
system_shutdown_timeout
tls_certificate
tls_prefer_openssl
tls_private_key
try_next_host_on_transport_error
use_lmtp
make_egress_pool
make_egress_pool
entries
name
ttl
make_egress_source
make_egress_source
ehlo_domain
ha_proxy_server
ha_proxy_source_address
name
remote_port
socks5_proxy_server
socks5_proxy_source_address
source_address
ttl
make_listener_domain
make_listener_domain
log_arf
log_oob
relay_from
relay_to
make_message
make_queue_config
make_queue_config
egress_pool
max_age
max_message_rate
max_retry_interval
protocol
reap_interval
refresh_interval
refresh_strategy
retry_interval
shrink_policy
strategy
timerwheel_tick_interval
make_throttle
memoize
on
read_dir
reject
set_config_monitor_globs
set_diagnostic_log_filter
set_httpinject_recipient_rate_limit
set_httpinject_threads
set_logging_threads
set_lruttl_cache_capacity
set_max_lua_context_age
set_max_lua_context_use_count
set_max_spare_lua_contexts
set_memory_hard_limit
set_memory_low_thresh
set_memory_soft_limit
set_qmaint_threads
set_ready_qmaint_threads
set_readyq_threads
set_smtpsrv_threads
set_spoolin_threads
sleep
spawn_task
start_esmtp_listener
start_esmtp_listener
allow_xclient
banner
batch_handling
client_timeout
data_buffer_size
data_processing_timeout
deferred_queue
deferred_spool
hostname
invalid_line_endings
line_length_hard_limit
listen
max_connections
max_message_size
max_messages_per_connection
max_recipients_per_message
meta
peer
relay_hosts
require_proxy_protocol
tls_certificate
tls_private_key
tls_required_client_ca
trace_headers
via
start_http_listener
start_http_listener
hostname
listen
request_body_limit
tls_certificate
tls_private_key
trusted_hosts
use_tls
start_proxy_listener
start_proxy_listener
hostname
listen
require_auth
timeout
tls_certificate
tls_private_key
use_splice
use_tls
toml_encode
toml_encode_pretty
toml_load
toml_parse
uncached_glob
version
module: kumo.aaa
module: kumo.aaa
auth_info
configure_acct_log
load_acl_map
make_access_control_list
make_http_url_resource
query_resource_access
set_acl_cache_ttl
set_check_cache_ttl
set_fall_back_to_acl_map
module: kumo.amqp
module: kumo.amqp
basic_publish
build_client
module: kumo.api.inject
module: kumo.api.inject
inject_v1
module: kumo.crypto
module: kumo.crypto
aes_decrypt_block
aes_encrypt_block
aws_sign_v4
module: kumo.digest
module: kumo.digest
crc32
hmac_sha1
hmac_sha224
hmac_sha256
hmac_sha384
hmac_sha512
sha1
sha224
sha256
sha384
sha3_256
sha3_384
sha3_512
sha512
sha512_256
module: kumo.dkim
module: kumo.dkim
ed25519_signer
rsa_sha256_signer
set_signing_threads
module: kumo.dns
module: kumo.dns
configure_resolver
configure_unbound_resolver
define_resolver
lookup_addr
lookup_mx
lookup_ptr
lookup_txt
ptr_host
rbl_lookup
reverse_ip
set_mx_concurrency_limit
set_mx_negative_cache_ttl
set_mx_timeout
module: kumo.encode
module: kumo.encode
base32_decode
base32_encode
base32_nopad_decode
base32_nopad_encode
base32hex_decode
base32hex_encode
base32hex_nopad_decode
base32hex_nopad_encode
base64_decode
base64_encode
base64_nopad_decode
base64_nopad_encode
base64url_decode
base64url_encode
base64url_nopad_decode
base64url_nopad_encode
hex_decode
hex_encode
module: kumo.cidr
module: kumo.cidr
make_map
module: kumo.domain_map
module: kumo.domain_map
new
module: kumo.file_type
module: kumo.file_type
from_bytes
from_extension
from_media_type
module: kumo.fs
module: kumo.fs
glob
open
read_dir
uncached_glob
module: kumo.http
module: kumo.http
Request
Response
build_client
build_url
connect_websocket
module: kumo.kafka
module: kumo.kafka
build_producer
module: kumo.mimepart
module: kumo.mimepart
builder
new_binary
new_html
new_multipart
new_text
new_text_plain
parse
module: kumo.mpsc
module: kumo.mpsc
define
module: kumo.regex_set_map
module: kumo.regex_set_map
new
module: kumo.secrets
module: kumo.secrets
load
module: kumo.serde
module: kumo.serde
json_encode
json_encode_pretty
json_load
json_parse
toml_encode
toml_encode_pretty
toml_load
toml_parse
yaml_encode
yaml_load
yaml_parse
module: kumo.shaping
module: kumo.shaping
load
module: kumo.spf
module: kumo.spf
check_host
check_msg
module: kumo.uuid
module: kumo.uuid
new_v1
new_v4
new_v6
new_v7
parse
module: redis
module: redis
open
module: regex
module: regex
compile
escape
module: sqlite
module: sqlite
open
module: string
module: string
eval_template
normalize_smtp_response
psl_domain
psl_suffix
replace
replacen
rsplit
rsplitn
split
split_ascii_whitespace
split_whitespace
splitn
trim
trim_end
trim_start
wrap
module: kumo.time
module: kumo.time
Time
TimeDelta
from_unix_timestamp
now
parse_duration
parse_rfc2822
parse_rfc3339
sleep
start_timer
with_ymd_hms
module: kumo.xfer
module: kumo.xfer
cancel_xfer
get_xfer_target
xfer
xfer_in_requeue
module: policy-extras.mail_auth
module: policy-extras.mail_auth
check
iprev
iprev_msg
module: tsa
module: tsa
configure_tsa_db_path
start_http_listener
object: address
object: address
domain
email
user
object: addressheader
object: addressheader
domain
email
list
name
user
object: authenticationresult
object: connectionmeta
object: headermap
object: headermap
append
bcc
cc
comments
content_disposition
content_id
content_transfer_encoding
content_type
from
get_first_named
iter
message_id
mime_version
prepend
references
remove_all_named
reply_to
resent_bcc
resent_cc
resent_from
resent_sender
sender
set_bcc
set_cc
set_comments
set_content_disposition
set_content_id
set_content_transfer_encoding
set_content_type
set_from
set_message_id
set_mime_version
set_references
set_reply_to
set_resent_bcc
set_resent_cc
set_resent_from
set_resent_sender
set_resent_to
set_sender
set_subject
set_to
subject
to
object: header
object: header
address_list
authentication_results
mailbox_list
message_id
message_id_list
mime_params
name
raw_value
unstructured
value
object: keysource
object: message
object: message
add_authentication_results
append_header
append_text_html
append_text_plain
arc_seal
arc_verify
check_fix_conformance
dkim_sign
dkim_verify
from_header
get_address_header
get_all_headers
get_all_named_header_values
get_data
get_first_named_header_value
get_meta
id
import_scheduling_header
import_x_headers
increment_num_attempts
num_attempts
parse_mime
parse_rfc3464
prepend_header
queue_name
recipient
recipient_list
remove_all_named_headers
remove_x_headers
save
sender
set_data
set_due
set_force_sync
set_meta
set_recipient
set_scheduling
set_sender
shrink
shrink_data
to_header
object: mimepart
object: mimepart
append_part
body
get_simple_structure
headers
id
rebuild
replace_body
events
events
get_acl_definition
get_egress_path_config
get_egress_pool
get_egress_source
get_listener_domain
get_queue_config
http_message_generated
http_server_validate_auth_basic
init
pre_init
proxy_server_auth_rfc1929
rebind_message
requeue_message
should_enqueue_log_record
shutdown_logging
smtp_client_rewrite_delivery_status
smtp_server_auth_plain
smtp_server_connection_accepted
smtp_server_data
smtp_server_ehlo
smtp_server_get_dynamic_parameters
smtp_server_mail_from
smtp_server_message_deferred_inject
smtp_server_message_received
smtp_server_rcpt_to
smtp_server_rewrite_response
smtp_server_split_transaction
spool_message_enumerated
throttle_insert_ready_queue
tsa_init
tsa_load_shaping_data
xfer_message_received
kumod HTTP API
kumod HTTP API
DELETE /api/admin/bounce/v1
GET /api/admin/bounce/v1
POST /api/admin/bounce/v1
POST /api/admin/bump-config-epoch
GET /api/admin/inspect-message/v1
GET /api/admin/inspect-sched-q/v1
GET /api/admin/memory/stats
GET /api/admin/ready-q-states/v1
POST /api/admin/rebind/v1
POST /api/admin/set_diagnostic_log_filter/v1
DELETE /api/admin/suspend-ready-q/v1
GET /api/admin/suspend-ready-q/v1
POST /api/admin/suspend-ready-q/v1
DELETE /api/admin/suspend/v1
GET /api/admin/suspend/v1
POST /api/admin/suspend/v1
GET /api/admin/trace-smtp-client/v1
GET /api/admin/trace-smtp-server/v1
POST /api/admin/xfer/cancel/v1
POST /api/admin/xfer/v1
GET /api/check-liveness/v1
POST /api/inject/v1
POST /api/xfer/inject/v1
GET /metrics.json
GET /metrics
schemas
schemas
Attachment
BounceV1CancelRequest
BounceV1ListEntry
BounceV1Request
BounceV1Response
Content
FromHeader
HttpTraceHeaders
InjectV1Request
InjectV1Response
InspectMessageV1Response
InspectQueueV1Response
MessageInformation
QueueState
ReadyQueueStateResponse
RebindV1Request
RebindV1Response
Recipient
SetDiagnosticFilterRequest
SpoolId
SuspendReadyQueueV1ListEntry
SuspendReadyQueueV1Request
SuspendV1CancelRequest
SuspendV1ListEntry
SuspendV1Request
SuspendV1Response
TemplateDialectWithSchema
TraceHeaders
XferCancelV1Request
XferCancelV1Response
XferProtocol
XferResponseV1
XferV1Request
XferV1Response
kumod Metrics
kumod Metrics
bind_failures
bounce_classify_latency
connection_count
connection_count_by_provider
connection_count_by_provider_and_pool
delayed_due_to_message_rate_throttle
delayed_due_to_ready_queue_full
delayed_due_to_throttle_insert_ready
deliver_message_latency_rollup
disk_free_bytes
disk_free_inodes
disk_free_inodes_percent
disk_free_percent
dkim_signer_cache_hit
dkim_signer_cache_lookup_count
dkim_signer_cache_miss
dkim_signer_creation
dkim_signer_key_cache_hit
dkim_signer_key_cache_lookup_count
dkim_signer_key_cache_miss
dkim_signer_key_fetch
dkim_signer_message_parse
dkim_signer_sign
dns_mx_resolve_cache_hit
dns_mx_resolve_cache_miss
dns_mx_resolve_in_progress
dns_mx_resolve_status_fail
dns_mx_resolve_status_ok
log_hook_backlog_count
log_submit_full
log_submit_latency
lruttl_cache_size
lruttl_error_count
lruttl_evict_count
lruttl_expire_count
lruttl_hit_count
lruttl_insert_count
lruttl_lookup_count
lruttl_miss_count
lruttl_populated_count
lruttl_stale_count
lruttl_waiting_populate
lua_count
lua_event_latency
lua_event_started
lua_load_count
lua_spare_count
memoize_cache_hit_count
memoize_cache_lookup_count
memoize_cache_miss_count
memoize_cache_populated_count
memory_limit
memory_low_count
memory_low_thresh
memory_over_limit_count
memory_usage
memory_usage_rust
message_count
message_data_load_latency
message_data_resident_count
message_meta_load_latency
message_meta_resident_count
message_save_latency
proxy_connection_failures
queue_insert_latency
queue_resolve_latency
queued_count_by_provider
queued_count_by_provider_and_pool
ready_count
ready_full
ready_queue_insert_latency
rocks_spool_cache_total
rocks_spool_mem_table_readers_total
rocks_spool_mem_table_total
rocks_spool_mem_table_unflushed
scheduled_by_domain
scheduled_by_tenant
scheduled_by_tenant_campaign
scheduled_count
scheduled_count_total
scheduled_queue_count
scheduled_queue_maintainer_count
smtp_server_rejections
smtpsrv_process_data_duration
smtpsrv_read_data_duration
smtpsrv_transaction_duration
thread_pool_parked
thread_pool_size
timeq_pop_interval
timeq_pop_latency
timeq_pop_tardiness
timeq_reinsert_tardiness
total_connection_count
total_connections_denied
total_messages_delivered
total_messages_delivered_by_provider
total_messages_delivered_by_provider_and_source
total_messages_fail
total_messages_fail_by_provider
total_messages_fail_by_provider_and_source
total_messages_received
total_messages_transfail
total_messages_transfail_by_provider
total_messages_transfail_by_provider_and_source
total_qmaint_runs
total_readyq_runs
user_lua_latency
Internal Rust API
Internal Rust API
bounce_classify
cidr_map
config
data_loader
dns_resolver
domain_map
duration_serde
kumo_address
kumo_api_types
kumo_chrono_helper
kumo_dkim
kumo_dmarc
kumo_log_types
kumo_prometheus
kumo_server_common
kumo_server_lifecycle
kumo_server_memory
kumo_server_runtime
kumo_spf
kumo_template
kumo_tls_helper
kumo_wrap
lruttl
maildir
mailparsing
message
mod_amqp
mod_aws_sigv4
mod_crypto
mod_digest
mod_dns_resolver
mod_encode
mod_file_type
mod_filesystem
mod_http
mod_kafka
mod_memoize
mod_mimepart
mod_mpsc
mod_redis
mod_regex
mod_serde
mod_smtp_response_normalize
mod_sqlite
mod_string
mod_time
mod_uuid
mta_sts
regex_set_map
rfc5321
spool
throttle
timeq
uuid_helper
version_info
Tags
FAQ
FAQ
Can I Migrate From Momentum (Ecelerity) to KumoMTA?
Can I Migrate From PowerMTA to KumoMTA?
Does KumoMTA Follow Secure Development Lifecycle (SDLC) Practices?
How do I flush a queue?
How Can I Get Help With KumoMTA?
How Can I Tell What Traffic Shaping Rules Apply To A Domain?
How do I skip IPv6 MX hosts for outbound SMTP?
How do I create an always-suspended queue?
How do I include multiple configuration files from a directory?
Is There a SaaS Version of KumoMTA?
How Can I Apply Multiple DKIM Signatures to a Message?
Why is KumoMTA Accepting Connections From Systems Not Listed in relay_hosts?
How do I resolve a `Permission Denied` error?
How Do I Configure POP3/IMAP?
How Do Shared Throttles Work for Small Connection Limits in a Cluster?
How Do I Avoid Having a Single Point of Failure?
How Can I Test TLS Injection?
What is KumoMTA?
Who is KumoMTA For?
Changelog
Changelog
Unreleased Changes in The Mainline
Release 2025.12.02-67ee9e96
Release 2025.10.06-5ec871ab
Release 2025.05.06-b29689af
Release 2025.03.19-1d3f1f67
Release 2025.01.29-833f82a8
Release 2025.01.23-7273d2bc
Release 2024.11.08-d383b033
Release 2024.09.02-c5476b89
Release 2024.06.10-84e84b89
Release 2023.12.28-63cde9c7
Release 2023.11.28-b5252a41
Release 2023.08.22-4d895015 - Automation
Release 2023.06.22-51b72a83
Release 2023.05.10-e88ad036 - Beta 2
Release 2023.03.31-36aa20de - Beta1
memory_low_thresh
Type: Gauge
low memory threshold measured in bytes.
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