Address book problem in GroupWise 5.5

Any e-mail addresses that include a period/full stop before the @ are being corrupted over night.

The next morning, john.smith@abc.com has become smith.john@abc.com

and mr.j.smith@abc.com has become smith.j.mr@abc.com

It is not just a display error, as the system does try to send the e-mails to the incorrect address.

Any ideas why, and any ideas on how to fix it?

Thanks,

Robin