Problem with sending email using smtp

I`ve got strange problem with sending email from GW2012 system. We`ve got about 30 accounts in example.com domain. On external server we`ve got configured some aliases pointing to the same example.com domain, for example alias1@example.com. The problem is that when user sends email using native GW2012 client to alias1@example.com everything works fine. But when he sends the same email to alias1@example.com using smtp he receives "550 no such recipient" error. It looks like GWIA treats example.com like it`s own domain and thats why the agent reports 550 error. How to fix it?