Actually I have not made the announcement yet but it is indeed possible now. Just define the DNS correctly (probably with a MX setting) and point it to mailcatch ip (87.98.134.65) and it shall work. Accounts are however not domain specific, maybe I'll change it if enough people contribute domains and the feature is desired.
Oh BTW, please notice me when a new domain points to MailCatch if you want to make it public, I'll add a section to list domains on the site.