We are running an Exchange 2013 server and I am trying to send to an email address starting with # to an external domain. After sending, I right away get a message back from Microsoft Outlook saying that it was undeliverable ('550 Requested action not taken: mailbox unavailable'). The generating server shows as our exchange server so it's like it is looking within the local Exchange server for this address.
When I check the message tracking in Exchange, it doesn't show in there, so I'm guessing that it is coming from Outlook not Exchange. I tried adding the email address to the Safe Recipients in the Junk Email settings in Outlook but that did not help.
I can send to this email address from Gmail and various other external email servers just not from our exchange server using Outlook 2010. Does anyone have any insight on what is causing this and how to fix it? Thank you.