[imp] adding default domain to addresses that don't expand?

[Liam Hoekenga]

> Try $_SESSION['imp']['maildomain'].

That worked just spiff.

Ok, so I've got a patch that creates a site-wide preference to govern 
adding the default maildomain to addresses that can't be 
automatically expanded (specifically, the ones that would normally 
return the "The field %s can't be expanded" / "Please resolve 
ambigious or invalid addresses" error message):

http://www.umich.edu/~liamr/horde/add_maildomain_to_unexpandable.diff

The other "More than one address found..." error is still handled as 
it was.

Liam