As part of UPC's sale of various assets to Vodafone, I am reliably informed that the domains upcmail.hu, chello.hu, and monornet.hu no longer host email users and should be considered retired. As of this writing, all still have MX records, but any attempts to mail those users will results in "550 5.1.1 MXIN501" errors.
Thanks to Mailkit's Jakub Olexa for this information!
