Issue with Exchange 2003 Server

I have an interesting issue with my Exchange 2003 Server. It was working fine until last Sunday. Late Sunday we noticed that we can not receive an e-mail from a few vendors. We did other testing and notice that we can receive an e-mail from any users except the one who are using Office 365 as their e-mail server.

Any Ideas what might of happened?

Thank you in advance,

Serge