Hello,
After upgrading to Exchange 2013 from 2010, it seems that if an external sender sends an email to several of our internal users and gets just one of the addresses wrong. It will reject the message and give #5.1.1 SMTP; 550 5.1.1 User unknown> #SMTP# for each recipient - not just the one wrong address.
The e-mail address you entered couldn't be found. Please check the recipient's e-mail address and try to resend the message. If the problem continues, please contact your helpdesk.
#5.1.1 SMTP; 550 5.1.1 User unknown> #SMTP#<u5:p></u5:p>
None of the legitimate users actually receive anything.
This is quite confusing for the sender, as they don't realise that they got just one address wrong, they think there is a problem with our mail server (which there kind of is).
We have Exchange 2013 CU3 installed on a single Windows 2012 server. CodeTwo Exchange rules 2013 are also installed for outgoing signatures, but I believe this problem has been happening before we added that.
Any help is appreciated.
Cheers
Tom