We have a setup as part of which secure web servers automatically send emails to various administrators to notify them that certain transactions have occurred. The messages are sent to addresses that cause them to be processed by our Zimbra mail server. The system has been working fine for a long time and continues to work well, apart from two recent exceptions.

About a week ago, the SMTP log on a secure web server indicated that a message had been sent, but the zimbra.log included no reference to it at all.

Then 3 days ago, again the web server SMTP log showed that two messages had been sent. This time there are corresponding entries in zimbra.log which indicate that the postfix queue manager begins to handle them.

May  5 15:10:37 mail postfix/qmgr[2517]: 41DFE30510:
from=<admin@caribbeanexams.com>, size=2523, nrcpt=1 (queue active)
May  5 15:10:37 mail postfix/qmgr[2517]: 02C933039B:
from=<info@eimacs.com>, size=4229, nrcpt=1 (queue active)
But that is the only mention of them in the log. In neither case is there a matching "removed" entry when the processing is complete. And both those queue ID#s appear 30 seconds later as ID#s for totally unrelated SMTP transactions.

None of the intended recipients of these two messages ever received them.

What else should I look for as I try to track down what is happening to the vanishing messages?