Fax Capability from Outlook broke

We are running Biscom server for our faxes.  The incoming faxes to our mailboxes are working just fine.  We switched the MX record last night and now people are unable to send faxes through Outlook.

We have an external domain that is different from our internal domain (something we are remedying in the near future), and our faxes are usually sent by inputing <fax number>@fax.<internaldomain>.com, then it goes to the fax server, and out.

So since all external mail now goes out 365, 365 is getting our faxnumber email address and sending it out to the internet because it does not match our external domain.

Thinking we need to setup an internal DNS entry pointing to the fax server with our external domain, then create a connector in 365 to push an email going to @fax.<externaldomain>.com back internally.  Not sure how to setup a connector to send it back in, but it sounds like that is what i need to do.

Any help with this would be greatly appreciated.

Jason.