When an outside user attempt to INITIATE a new email, the messages are bounced back with an undeliverable error (error varies, ususally "mailbox is unavailable"). When the outside user REPLIES to an email message, they comes in and is deliveeredwith no problems.Email message sent from a GMAIL account get the following bounce back message: Google tried to deliver your message, but it was rejected by the recipient domain. We recommend contacting the other email provider for further information about the cause of this error. The error that the other server returned was: 550 550 Requested action not taken: mailbox unavailable (state 18).Sometimes, If the user starts the subject line with "FWD:" or "RE:" the messages will be delivered.Any ideas as to where the problem resides

If I had to make a wild guess, I would think that when someone outside is sending you mail, they are using the wrong e-mail address. If you want to test it, send me a test e-mail from that system. You can send to jim (at) cta.net Just make sure you put some text in to the message body, otherwise it will be rejected as spam.Jim McBee - Blog - http://mostlyexchange.blogspot.com

Verified repeatedly that the email addresses are correct, yet problem remains.

Are you using an email hosting service or another gateway or are messages delivered directly to the Exchange Server from the Internet?

Exchange 2007 on Windows 2003 64-bit server.All Service Packs installed and updated.We user Forefront for Exchange for Virus filtering and some "Keyword" "Subject" filterering.Messages are delivered directly to the Exchange server.No hosting service...Thanks

Ok, in that case, enable SMTP logging n the Exchange side and have send some test messages and then check the logs to see if anything helps to troubleshoot.http://technet.microsoft.com/en-us/library/bb124531.aspx"How to Configure Protocol Logging"

I went into the Exchange console and disablethe re-enabled all of the Spam filtering and that seems to have corrected the problem.Who'd a thunk it.

Sounds like there may have been an issue with recipient filtering. Glad you fixed it.James Chong (MVP) MCITP | EA | EMA; MCSE | M+, S+ Security+, Project+, ITIL msexchangetips.blogspot.com