1. Time/date/DNS/ or Gateway settings on the 7775 is wrong
2. 7775 has a Fiery attached which would make the setup entirely different than the others.
3. 7775 has a firmware not compatible with the mail server (this would be the case if it has not ever worked to the server, not if it just stopped working "for no reason today"), The latest firmware is 061.090.227.19300 and should fix most issues with cmpatibility
The actual fault code would be more helpful than "won't work"