I have the same problem as several others who have reported this problem. An outside sender cannot send to us, and this sender has an apostrophe in their email address .. I'm running Exchange 2000 under W2K with a Watchguard FB1000. All of our email goes to an outside service that supplies spam filtering; anything not filtered out is passed through to our firewall. The firewall is set to use SMTP proxy for incoming, and I've set that to allow apostrophes. The spam service reports the 553 message to the outside sender, and reports that it came from our server. In other words, the spam service is not trapping these emails. It looks like Exchange is rejecting them, not the firewall. An earlier response to someone who had this problem with Exchange 5.5 suggested upgrading to Exchange 2000, but I have the same problem. Is there a setting in Exchange that I've missed?