Symptoms:
Your email vacation notices are not being received by recipients. If you look in your server's mail.log you see that outbound vacation notices are showing a rejection by your relay server...
postfix/cleanup[26483]: 130C3DBBB7F: message-id=<cmu-sieve-26492-1285246620-0@mailserver.somewhere.co.uk>
postfix/smtpd[26486]: disconnect from localhost[127.0.0.1]
postfix/qmgr[26444]: 130C3DBBB7F: from=<>, size=1174, nrcpt=1 (queue active)
postfix/smtp[26484]: EF8D6DBBB78: to=<redpaw@example.co.uk>, relay=127.0.0.1[127.0.0.1]:10024, delay=0.11, delays=0/0/0/0.1, dsn=2.0.0, status=sent (250 2.0.0 Ok: queued as 130C3DBBB7F)
postfix/qmgr[26444]: EF8D6DBBB78: removed
postfix/smtp[26496]: 130C3DBBB7F: to=<redpaw@example.co.uk>, relay=smtp.ntlworld.com[81.103.221.11]:25, delay=2.2, delays=0/0.01/2.1/0.04, dsn=5.0.0, status=bounced (host smtp.ntlworld.com[81.103.221.11] said: 550 relaying mail to example.co.uk is not allowed (in reply to RCPT TO command))
Note the "from=<>" address which the vacation message is being tagged with.
Reason:
In short, the relay server is rejecting the blank (NULL) From: address as likely spam.
Any Fix?
Nope! In 10.5 Server (or older) the NULL address is hard coded into the Cyrus mail server for its initial communication with the recipient or relay server.
Why Does It Use A NULL From Address?
A vacation response is a form of "Notification Message". It's a type of non-delivery message. Lets go to source documentation from Cyrus mail server WIKI...
Why Does Cyrus Set The MAIL FROM Address of the Sender of Vacation Responses to '<>'?
"Because many of the Sieve features are autoresponders, Cyrus tries to be a good network citizen by adhering to the appropriate RFCs. From RFC 2821:
"...SMTP servers MUST NOT send notification messages about problems transporting notification messages. One way to prevent loops in error reporting is to specify a null reverse-path in the MAIL command of a notification message. When such a message is transmitted the reverse-path MUST be set to null (see section 1.5.5 for additional discussion).
So What Can I Do?
You could contact the provider of your relay service as this is an anti-spam measure which they have implemented. You will not be the only customer being caught by this. If they will not change their policy then you either need to find another relay service (e.g., MailHop Outbound from DynDNS.com) or remove the relay from your server and send email directly out yourself. However, the latter may not be an option if your IP address is one of a blacklisted block.
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