I might just be using the wrong keywords on Google, but what I have in mind is:
--------------------------------
| What is your mailbox? |
| |
| [ ]@mail.example.org |
| |
| [OK] [Cancel] |
--------------------------------
The idea being that the input field is followed right behind by the mail server name, to help avoid instances where:
If I don't make it follow right behind, the user puts in the whole thing,
If I default it to "[email protected]" they delete the server name.
In either case, it's not too big a deal, as I will know that if the returned value does have the server, to remove it and if doesn't, I know what server it belongs to, but I think this visual aid will be a better user experience and lower the amount of validation worries I tend to get.