Last night I noticed our customer backup MX's mailqueue was quickly filling with mails to a certain domain, although the destination mailserver was rechable just fine.
After some searching I found out that there was a (spam) mail to some 100 recipient addresses in the queue, and the destination domain's combination of Firewall I and Interscan Viruswall fucked up on their recipient limitation: After 50 recipients, their server returned 400 class errors on all following commands, even after an RSET, so that our systm couldn't deliver this specific mail and also no other that came later in the queue. Great.
Fortunately, sendmail knows a SMTP_MAILER_MAXRCPTS define, which will break up delivery of mails with more than the maximum of recipients into several sessions (lowered confMAX_RCPTS_PER_MESSAGE too on that occasion - was previously unlimited because of the historical reason that some customers used the MX as outgoing mail server for some mail clients that couldn't limit the number of recipients in a session for CC orgies).
After some searching I found out that there was a (spam) mail to some 100 recipient addresses in the queue, and the destination domain's combination of Firewall I and Interscan Viruswall fucked up on their recipient limitation: After 50 recipients, their server returned 400 class errors on all following commands, even after an RSET, so that our systm couldn't deliver this specific mail and also no other that came later in the queue. Great.
Fortunately, sendmail knows a SMTP_MAILER_MAXRCPTS define, which will break up delivery of mails with more than the maximum of recipients into several sessions (lowered confMAX_RCPTS_PER_MESSAGE too on that occasion - was previously unlimited because of the historical reason that some customers used the MX as outgoing mail server for some mail clients that couldn't limit the number of recipients in a session for CC orgies).