Someone sends with my username

Hi.

We have Exchange Server 2010 and Spamtitan installed (mail edge).Someone sends mail from my username to another domain username.We set Relay ip only our internal mail server ip adrress in Spamtitan and In the header of message there is ip adrress of sender and it is from another country.I tested it with online tools and nothing success.How he did it?

Site is emkei.cz