Question about Outlook Express

I have a custom domain that is set to auto forward messages to my outlook.com/Hotmail address.  It has been working for well over a year, but suddenly stopped working yesterday afternoon.  I have checked my junk email, safe/blocked senders, and email rules.  But nothing has changed and nothing warrants causing an issue like this.  It seems as though something has gotten blacklisted or something else, but I have been unable to trace down the issue.  Any assistance would be greatly appreciated!