Hi Leon, just saw that thread. Unfortunately you can't do much about this. Most of the time it's not that your account has been hacked or anything when people are using your email address in the 'from' field.
There are two flavours out there.
1_ Mail spoofing. You get the occasional email from
leon@domain.com (your email address). That's what your mail client says but if you check the mail header you'll see that the email doesn't even come from you. To stop this there is a system that does a very 'relaxed' check called a SPF record. It's a line of text in the zone file of your domain with the web host. But it is not very effective.
2_ Back scattering. This one is a real pain. You get 100s of bounced emails coming from various domains. It's not your fault either. It comes from compromised SMTP servers or unsecured web forms. Automated bots use
leon@domain.com in the fields then spam the web. The messages that are undeliverable bounce back to
leon@domain.com and you get all the failed delivery notices.