Domain directive
max-smtp-out
Limit simultaneous outbound SMTP connections for a destination domain.
PowerMTAv4.1 frameworkProduction reference
Production caution: Start conservatively and tune from current responses, queue age, and reputation.
Purpose
Limit simultaneous outbound SMTP connections for a destination domain. This reference focuses on operational intent, placement, validation, and failure modes rather than presenting a value as universally safe.
Example syntax
max-smtp-out 3Placement and scope
The effective scope depends on where the directive is placed: global, source, SMTP user, virtual MTA, pool, or destination domain. Keep related controls together and comment the business purpose of each override.
Verification procedure
- Back up the active configuration and included files.
- Confirm the directive is supported by the installed PowerMTA release.
- Validate every referenced VMTA, pool, file, hostname, IP address, selector, and pattern list.
- Reload safely and watch the main log for parser or runtime errors.
- Send a controlled test and inspect the exact source IP, EHLO, headers, TLS state, and SMTP response.
Common failure modes
- Placing the directive in a scope where it is ignored or overridden.
- Referencing an object that is misspelled or not defined.
- Copying syntax from a different PowerMTA version.
- Using aggressive values without measuring provider acceptance and queue age.