Change your email account and notify your correspondents of same.

The bottom line is don't use public email accounts such as those offered by gmail, yahoo, livemail, etc.
They have proven to be not secure and hazardous to your well being.
They work well for throwaway stuff, but I would never use one of them to send mail to anyone with whom I have a commercial interest or anyone I personally know. It isn't worth the risk as you have discovered.