EGroupware is doing absolutely nothing here.
Horde_Smtp used PHP function gethostname() to determine the name of the host it’s running on and sends that to the SMTP server.
Maybe you missed to configure that correctly on your webserver.
To check you can run e.g.:
RalfsMac:~ ralf$ php -r 'var_dump(gethostname());'
Command line code:1:
Or disable the check on your MTA.