Compose... showing IP address in sender email address instead of domain

[hellfyr]

If a user logs in using just their account name (without the domain.dom) then when they try and compose a message
the SENDER is listed as
    <username@ipaddress>

If however they signon with their full domain it shows the correct email address during a compose message
     <username@domain.dom>

With the $config['user_domain']  setup why wouldn't it append this as the domain since it allows the user to login this way ?
Or is their another setting for that ?

Thanks,

Rod S
   

[JohnDoh]

There is another config option 'mail_domain' you need to set that one too.