URGENT Help - 2 Domains

[Weinstein]

I just installed Zimbra 6 on Fedora and all seems well.

I can sent and recieve mails (almost)

the second domain sends emails from mail.firstdomain.com which some email servers do not like.

How do I make the second domain send from mail.seconddomain.com.

Both share the same ip address

Thanks

[phoenix]

It shouldn't make any difference that you send mail from a second domain on your server as long as the DNS records (including reverse DNS) are valid and correct for the primary domain. You might also want to investigate using SPF for your server & domain.