exchange won't accept emails-User unknown 550 5.1.1 error

[ladyoclass1]

I had to reinstall IIS and Exchange this weekend because it was restarting overnight and then some functions of the GFI anti-spam would fail. Now Exchange won't accept any mail and I get the User unknown 550 5.1.1 error.
The server is accepting email for multiple domains and if I send to the server domain itself, I don't get a bounce back although the mail never arrives either. If I send to one of the other domains, I get the 550 5.1.1 error.
How can I troubleshoot this and what would be the first place to look? My phone is ringing off the hook with upset clients.
Need I say I am very new to exchange.

Any help would be greatly appreciated. I am desperate to get this working again.

Thanks.

 

[DJCopa]

Hello ladyoclass1.

Hope your day is getting better!

We have had this error bounce back to us, when emailing certain domains. I believe it is down to your email server thinking that someone is relaying through it. Check the setting on your Exchange server, to 'Allow computers which successfully authenticate to relay'.
This setting can found in Exchange System Admin/Admin Groups/Servers/Protocols/SMTP/Default Virtual Server.
Right Click on this, and select Properties. Relay Restrictions can be found here - The tick box can be selected for the above setting.

Hope this helps and Good Luck!