Ideas? loads, give us some more detail and we might be able to help ....
Something like:
realdomain,vdomain1,vdomain2,vdomain3
usera@realdomain works fine
usera@vdomain1 and vdomain2 works fine
usera@vdomain3 does not work
-or-
userb@realdomain can send mail to usera@realdomain and userc@realdomain and userc@vdomain1 but not usera@vdomain1
-or-
userd@somewhere-else can send mail to usera@vdomain1 and userb@vdomain1 and userb@vdomain2 but not usera@vdomain2
-or-
usera@realdomain and userb@realdomain work for mail to and from everywhere.
usera@allvdomains and userb@allvdomains don't work from outside at all
There's lots of permutations covered by
now everything works except sending email to a user in one of my virtual domains
Anything from dns entries to client software to server config to choice of usernames ....
David.