Mail.app: New mail sound but no new mail

[Vandal.]

Hey guys,

I recently noticed that mail.app is playing a new mail sound without any new emails being displayed in the inbox. Testing this by sending emails to myself I discovered that this problem only exists if I send these from a certain account in mail.app. A second after the test email I get the "bing", but there's no mail whatsoever.

What do you think, is this a server side issue or some sort of bug in mail.app?

Any help would be greatly appreciated.

Thanks,
V