It baffles me too. I hoped to find that the mail.box was corrupted, but alas...
Another option is to test with debugging enabled. It will generate a LOT of data in the log files. Whether it works from server to server I don't know, so be careful.
From
http://www.advancedclp.com/cool_tips.html:
How do you see the NRPC traffic that flows between the Notes client and the Domino server?
Use the client NOTES.INI parameter CLIENT_CLOCK=1 then restart your client. A debug window will appear and the NRPC traffic wil be shown in the window.
Another interesting document:
http://www.mmi-consult.de/FAQ/dx/MailroutingT-shootingFinal.pdf/$file/MailroutingT-shootingFinal.pdf