Second server cannot send email to own domain (hosted on first server)

[benbalbo]

I'm responding to this thread as it's related.

When I email a user on my second server this shows up in the logs:

Apr  4 22:00:24 server2 postfix/cleanup[427]: 60D9320CE04D: message-id=<4433236D.60100@benbalbo.com>
Apr  4 22:00:24 server2 postfix/qmgr[31863]: 60D9320CE04D: from=<ben@benbalbo.com>, size=2322, nrcpt=1 (queue active)
Apr  4 22:00:26 server2 postfix/smtp[31865]: setting up TLS connection to mail.main-domain.com
Apr  4 22:00:26 server2 postfix/smtp[31865]: verify error:num=18:self signed certificate
Apr  4 22:00:26 server2 postfix/smtp[31865]: Peer verification: CommonName in certificate does not match:  != mail.main-domain.com
Apr  4 22:00:27 server2 postfix/smtp[31865]: Peer certificate could not be verified
Apr  4 22:00:27 server2 postfix/smtp[31865]: 60D9320CE04D: to=<web5_user@main-domain.com>, orig_to=<user@client-domain.com>, relay=mail.main-domain.com[***.**.**.**], delay=3, status=bounced (host mail.main-domain.com[***.**.**.**] said: 550 <web5_user@main-domain.com>: Recipient address rejected: User unknown in local recipient table (in reply to RCPT TO command))
Apr  4 22:00:27 server2 postfix/cleanup[427]: F3C2D20CE078: message-id=<20060405020027.F3C2D20CE078@server2.main-domain.com>
Apr  4 22:00:28 server2 postfix/qmgr[31863]: F3C2D20CE078: from=<>, size=4391, nrcpt=1 (queue active)
Apr  4 22:00:28 server2 postfix/qmgr[31863]: 60D9320CE04D: removed
Apr  4 22:00:29 server2 postfix/smtp[31865]: setting up TLS connection to main-domain.com
Apr  4 22:00:29 server2 postfix/smtp[31865]: verify error:num=18:self signed certificate
Apr  4 22:00:29 server2 postfix/smtp[31865]: Peer verification: CommonName in certificate does not match:  != main-domain.com
Apr  4 22:00:30 server2 postfix/smtp[31865]: Peer certificate could not be verified
Apr  4 22:00:31 server2 postfix/smtp[31865]: F3C2D20CE078: to=<ben@benbalbo.com>, relay=main-domain.com[***.**.**.**], delay=4, status=sent (250 Ok: queued as 079B9529C03B)
Apr  4 22:00:31 server2 postfix/qmgr[31863]: F3C2D20CE078: removed

English translation:

I send an email to my client, they are hosted on server2. Server2 receives the request and rewrites to web5_user@main-domain.com, which is server1, so relays it. I get a undeliverable telling me the email address doesn't exist on the server1.

I'm presuming there's a way to change the ispconfig setup to treat the server as server2.main-domain.com, and hence rewrite the email address to web5_user@server2.main-domain.com?

I was going to try changing (in Management > Server > Settings > Server Tab) from:
Hostname: server2
Domain: main-domain.com

to:
Hostname:
Domain: server2.main-domain.com

I just want to see what the preferred method of making this server deliver mail locally is, rather than changing these settings and potentially bringing the server down horribly.

Thanks in advance!

Ben