External users can't send mail to their own domain

[alex]

Hello list,

I have a kolab 2 server setup and it is working very well for all users 
in the office. I have two external users who cannot send mail to the 
domain. The users have accounts on the kolab server. They are using 
their own smtp server, but their imap account is on the kolab server.

Below is the message I get in the Mail delivery failed email:

This message was created automatically by mail delivery software.

A message that you sent could not be delivered to one or more of its
recipients. This is a permanent error. The following address(es) failed:

  don at mydomain.net
    SMTP error from remote mailer after RCPT TO:<don at mydomain.net>:
    host mail.mydomain.net [4.35.170.39]: 554 <alex at mydomain.net>:
    Sender address rejected: Invalid sender


Thanks,

Alex