I have currently 2 domains, lets call them:
main.com and other.com.
The majority of my users need to be user@other.com.

The configuration I have is:

user@main.com, with an alias user@other.com pointing to user@main.com.

This works mostly, however; if a user sends an email which is then replied to... and the user replies to the reply... his sent from address will change from user@other.com to user@main.com. I need this not to happen. Also, all GAL addresses show as user@main.com, and they need to show user@other.com.

I set it up this way to migrate users, but now I have no need for these users to have any reference to main.com. It's fine that they use server.main.com to access the web client, but from then on, need them only ever to show user@other.

Is there something I've missed in setting up the aliases, or do I just need to move these users to the "other" domain?

I've looked for a good while, but have not found info on this exact issue...

Look forward to any suggestions, or a pointer to a document which addresses these issues that I may have missed.