Outgoing Mail: Postfix states sometimes "User unknown in virtual alias table"

[bitrockers]

Hi everybody.

I've run into a strange problem with outgoing mail sent by the support suite. When supportsuite is sending an autoresponder mail or on replying to a ticket, sometimes I receive a bounce message from the recipients mailserver:

".. User unknown in virtual alias table"

The strange thing is, that the recipients emailadress is a valid one. Did anybody encountered a similar problem.

Too clarify the problem, I'll describe the situation:

Supportsuite is installed on Linux Debian Server running PHP 5.2. Mailer is postfix. Supportsuite is configured to use phps mail() function. I've tested with serveral emailadresses, say:

working@domain-a.com
not-working@domain-b.com

When supportsuite is sending emails to working@domain-a.com everything is ok. When the recipient is not-working@domain-b.com, I receive a bounce message stating "User unknown in virtual alias table".

Ok, you could think something is wrong on the recipients side (maybe user is really unknown). BUT sending emails to not-working@domain-b.com

a) by independent ISPs SMTP server does work

b) by an independent php script on the same server (where supportsuite is installed) using the mail()-function, does work, too.

c) by shell-command sendmail ( again same server where supportsuite is installed), does work.



So, I have no clue why sending from supportsuite creates a bounce message by the server. As I have no idea what exactly supportsuite is doing, I'm actually stuck with that thing.

Any ideas or suggestions where to look for the problem are appreciated.

Greetings
Christian