Recently had my e-mail hacked - it was being used to send out spam mail and was locked by virgin media - account reopened, password changed and off we go.
I'm now receiving mail send failure notices for Russian email addresses, though nothing appears in my sent items and emails from 2 or 3 people in Russia all with 'links' on them, needless to say I haven't clicked on any of the links.
They may simply be delayed failure messages from the time that your account was being used by the spammers. You would not see such sent emails in your sent items as the spammers were using your credentials not your account directly.
Provided you have since changed your password and security answers and ensured that your devices are clear of any malware or key tracking viruses your account should now be secure.