[Dovecot] sender address of sieve vacation script

[Wolfgang Burger]

Hi,
not entirerly sure wether this is the right place to ask. Sorry if not.
I´m running dovecot on a Mac OS X Server (10.6.4).
Everything works fine.
When I try to enable a vacation message, I get the following error in  
the log:
delays=0/0.01/0.09/0.1, dsn=5.0.0, status=bounced (host XXX[x.x.x.x]  
said: 553 Mail to <name at domain.tld> not allowed; missing domain name  
in MAIL FROM (in reply to RCPT TO command))

Where host is the mail relay. The logs on the mail relay show matching  
entries.
Sending mails is generally working.

I´ve sniffed the network traffic and the FROM Field in the SMTP  
transfer contains only the following text:
<>

I´ve allready set the value of myorigin in /private/etc/postfix/ 
main.cf but to no avail.
Can somebody tell me where I can set the sender address of the  
vacation mail?
Or is there something else wrong?

Thank you very much.
Best regards
Wolfgang