Running Lotus Notes on Linux thanks to Ketan's fabulous HOWTOs? Great!
Did your sysadmin decide it was time for you to use a new mail server? Drat.
For some reason, the Notes client is smart enough to switch mail servers over automatically on OSX and Windows. If you can't connect to your old server anymore, reconfigure your client here:
File -> Lotus Notes Preferences -> Client Reconfiguration Wizard
Tada! All fixed.