On an IPhone, when I reply to an internal user, the email does not get delivered. Checking in sent items|status shows "transfer failed". Replying to external email works fine.

GW returns error message " Item to user(s) at <userlastname> was undeliverable. So, it appears that the mail is attempting to deliver replies to internal user(s) with the lastname only? Not sure...

Our naming convention is first initial.lastname for the mail boxes. Email properties show FirstInitial.lastname.POSTOFFICE.DOMAIN.

Our internet addressing rules allow this as well as firstname.lastname at domain name.

If I use my IMAP account to the GWIA, everything works fine.