The SMTP RFC 5321 (see RFC 5321) specifies that a sending server must queue messages that cannot be directly delivered due to a temporary failure at the receiving end.
Temporary Issues and Queuing
The rationale behind this is that if there is a temporary issue with the email infrastructure, emails will not be bounced immediately. Instead, they are queued on the sending server(s) and automatically retried for delivery.
Handling Downtime
In the event of downtime for the destination mail server, messages will only be accepted for delivery by the filter cluster if the recipient is known to be valid. Valid destination recipients are cached for up to 96 hours.
Message Bounce Policy
If a destination server cannot be reached for 4 days, all messages will be bounced, and no new emails will be accepted or queued until the destination server is back online. These 4 days is established to comply with the SMTP RFC.
Importance of Timely Awareness
The reason for this 4-day limit is to ensure that the sender is aware that the delivery of their message has been failing for this duration, allowing them to try to contact the recipient through alternative means.
Gotchas to Avoid
- Regularly monitor your email delivery status to catch issues early.
- Consider alternative contact methods if delivery fails repeatedly over several days.
