We have a bunch of customers with French accents in their names that causes stranges issues.
I know Mitel will address that in a future release.
Never heard of issues related with UM forwards. Maybe you could try to download Nupoint UM logs to find out more. Try to SFTP into your server and download text files from /usr/vm/log/msblogs/
Just a though... I know Nupoint is trying to find UM messages in you mailbox by seeking (using IMAP) all messages that contains "nupointmsgid" in the subject line, rejecting all messages that begins with "FWD" or "RE". In your case, French Outlook forwards are using "Tr" instead of "FWD".
I think the best thing to do is to call your VAR... They'll be able to find out if this issue is already fixed or they'll be able to report that to Mitel.
Hope that helps!