According to the sources, the CMUSieve plugin will refuse to autorespond in the following situations:
Thanks Stephan. It is indeed the CMU Sieve plugin (sorry for not pointing that out). I saw those conditions the wiki somewhere and I don't believe any of the conditions are met, though perhaps I need to check the mail headers more to be absolutely certain. In most cases in which vacation fails, the sender envelope address is not known. This can be caused by a missing return-path header, but (when
Phill Edwards wrote: the return-path header is not used) also by a missing -f parameter for deliver. So it is not always a message header problem. Alternatively, the message could have been sent with a null sender address '<>', but that is usually done only by automated e-mail responders.
Is the Dovecot Sieve plugin more lenient/easier to use and would it be worth giving it a try? I can't remember the exact version of Dovecot I'm running until I go home to check, but I'm pretty sure it's < 1.2 which is why I'm using CMU Sieve. It will at least provide you with an informative log message on why no vacation response is sent. Better log/error messages for administrators and users is one of the new implementation's main goals. So, it will most probably help you finding the issue. It is however not more lenient regarding the restrictions of the vacation action and, being the author, I cannot really claim that the new Sieve is easier to use than the old CMUSieve. It should be anyway :)
Regards,
Stephan