I tested this on 2 machines (same software). I also have e-mail disabled on the forum. If I try to register as a user, the software hangs for about 15 to 30 seconds and then tells me "The entered e-mail domain has no valid MX record". I made sure that the e-mail address I was using had a valid MX record, and even tried other e-mail addresses with the same result. I am unable to test on a production server, however I never had this problem before. The only workaround I can think of is to disable the MX record check via the ACP, but this is definitely not the intended solution.