Most email programs can be set to use any address in the "From:" header.

How to do this for Thunderbird is described in another question in this FAQ section.

We're working on putting up more details on the site about how to do this with other email programs. Meanwhile, contact us if you have trouble setting this up with your email program.

In the Neomailbox webmail system, you can add From: addresses from the '"Preferences" screen, by creating new address in the "Addresses:" section.