'Reply to All' is appending a trailing '.' to recipients, any ideas why?

One of our users has a mailbox issue where if he hits ‘reply to all’, the last recipient in the list gets a ‘.’ appended to the end of their email address.

For example, you get a message in with jill@ibm.com and jack.ibm.com in the CC fields. Hitting reply to all gives you jill@ibm.com, jack@ibm.com. in the TO field.

I’m stumped. I don’t see anything obviously different with his client setup.

Any ideas?