Sometimes you can randomly receive an email stating “the domain … has exceeded the max defers” which indicates that too many recent emails have been sent to non-existent email addresses. It is normally a sign of spam being sent from your account.
The easy fix is to change your mail password and run a virus scanner on your PC to make sure you have no dangerous files on your PC. These two steps normally fix any problem.
However, sometimes it can be that your account is not compromised and it is due to forwarding rules. Essentially, spam is sent to your account and then forwarded to another email address which recognises it as spam and rejects it.