550 5.7.1 Unable to relay for user@domain.co.uk (incoming email)

[TanZail]

There is a problem with my system. I am working on a new server. I had configured it properly and created around 20 users. Later on I recreated the AD accounts due to some issue. I migrated them to the Exchange boxes with xmerge. All mail test were properly. It was working fine. Later on switch the router firewall to a new server ip. But after switching the same when I try to send mails from client system I am getting a error. It is some PERM_FAILURE: SMTP Error (state 13): 550 5.7.1 Unable to relay for
user@domain.co.uk.

[Arzhela]

It looks like the SMTP firewall is the cause of problem. It is rejecting the message and due to which you are not able to work. The SMTP relay in the firewall and it is necessary that you must reconfigure the domain accept mails for it. It is better if you can give more detailed description of your system and then I hope there will be some solutions that would work.