So i've been seeing a new error for some reason, whenever an outside user tries to send e-mail to a particular user. This users mailbox was actually working fine for months; however whenever someone from an outside domain tries to email him now, they always get a "delivery failed" error right away. Also, some failed users sending the email get the following error code as well: "smtp;550 5.1.1 : Recipient address rejected: User unknown in virtual mailbox table"
What's weird is, everyone else's email works fine, that was created before this user. Also, I tried creating a new mailbox/user for testing purposes, and I get the same error right away whenever I try and send any newly created user a test email from outside the domain (IE: hotmail, etc).
I dug around exchange and everything looks fine. Even tried running MXToolbox and the SMTP tests all clear fine. I'm thinking it might have to do with our External Web Filter company: SPAM HERO; however not positive because this error just started occurring. Has anyone ever seen this before and/or assume it can only be a problem with SPAM HERO? Thanks in advance