Problem with Exchange 2013 mail flow

Hi all,

We have recently migrated from a local provider email hosting plan to  a new in premises exchange 2013 STD SP1 installation and we are having an issue while sending some emails out.

Most of our communication is ok, but on some specific customers we send emails we get NDR with a "mailbox unavailable result", which can't be right because we used to (and we are still able to)receive emails from them.

Any Ideas on what we need to do to solve the issue.

We have currently disabled TLS on our send connector to check whether this solves the issue with no success.

more details on our installation is we are using a single server for our exchange 2013 (mailbox & Client access roles), installed on a Windows 2012 Server, and are using a mixture of Outlook 2007, 2010 and 2013 as Clients. The problem has nothing to do with the clients as we get the same NDR's from all version of outlook