How to stop someone from sending emails from my domain using hotmail

Solution 1:

No, you cannot. Hotmail will have to deal with it themselves. However because this is an organization and his secondary email clearly identifies the organization address, the folks at Hotmail will have no doubts about your report and the problem should be dealt with swiftly.

Solution 2:

Your organization could set up SPF (Sender Policy Framework). SPF is a system where the owner of a domain can specify which servers are entitled to send email using that domain in the From address. For instance, I used the Beveridge tester to query superuser.com, and it turns out that they have specified the Google mailhosts only to send mail as [email protected]. If your Hotmail-using ex-employee tried to send mail from [email protected], SPF-compliant servers would either reject the mail or assign it a higher spam score.

Note that details of setting up SPF are ServerFault stuff, not SuperUser. In fact, your question is arguably a ServerFault question in the first place.

Solution 3:

If he is costing you money, you can sue him for damages. If not, talk to an attorney to get an injunction and also get attorney’s fees, even tough just a letter by him or her may just do the trick.