I shall do what I can to help with this, especially as this is a security risk and needs to be dealt with quickly and effectively.
The letters and text messages sent by our Email Security teams are in reaction to alerts of compromised email accounts. The tell is usually the amount of mail sent out (usually spam) and the originating IP address(es). These emails are authenticated - which means that the email address in question has been compromised. Your email address is not being spoofed (where the sent from/receipt to address has been altered to mimic yours) but your actual credentials are being used. For instance my tests of your SMTP logs indicated that spam, sent from your account, originated from IP's linked to Serbia and Pakistan.
The information contained in the letter has be followed exactly.
You must set a new, unique password, that has not been used previously.
You must ensure that all of your Internet connected devices (pc's, laptops, tablets, phones etc) are scanned regularly with up-to-date security software, as one of these is likely infected with a key logger or trojan for example that is retrieving your personal data.
If not dealt with effectively the issue will recur and Email Security will reach out to you again. Please click herefor an overview of the information that would have been contained in the letters from Email Security.
Let us know if you require any further help with this or have any related questions