I have been trying to get email up and running for several days now and I'm still fighting it. I hate to compare this great product to cpanel, but with cpanel, email just works - why is it so hard here?
There are so many posts here in the forums that it is difficult to see the forest through the trees - needless to say, I'm a bit frustrated, I finally was able to get it to work but it shouldn't be so difficult.
Some advice for those of you who find this post in the future. Use the command line to create users and forwarding, I find it much easier and it sets up the accounts properly. Specifically, postfix doesn't like an "@" symbol in the user name, so when you use the command line, two users are created, firstname.lastname@example.org and joe-smith.com. Use the "-" account for logging in purposes.
When you create a forwarder and you have a user name of joe-smith.com, the "from" field should just be "joe" and not "joe-smith.com" Why? I have no idea but that is what it took for it too work.
I really do like Virtualmin/Webmin but the email functionality leaves a lot to be desired.