To Brian:
"Can you then write to them as an external user?? Like I am outside the domain?? "
Yes, simply disable the "Internal use only" setting.
Otherwise, you can use the option that Michael Gralke Augusto described above, which is to send a "system admin" mail to users from the web admin interface.
Gabriele Maoret - Head of SysAdmins at SERSIS
Currently manages 6 SmarterMail installations (1 in the cloud for SERSIS which provides services to a few hundred third-party email domains + 5 on-premise for customers who prefer to have their mail server in-house)