Click envelope icon above.
Click start new conversation.
Type recipient name in the appropriate field.
Spam.
Honestly, writing messages to people on their profiles has always been a stupid design decision, however the one problem we might have is that there is a number of conversations limit and participants number limit.
That is a long standing problem here and I am geussing a xenforo configuration.
Or for all I know it is another stupid bug no one reported.
@LordSunhawk mind letting us have more than 5 e-mail chains going on?