How not to set up your DNS (part 20)
I call this one the case of the non-redundant redundant MX; it's much like the first time except more thorough:
; sdig mx mumble.utoronto.ca. 0 mail.mumble.utoronto.ca. 0 jackson.mumble.utoronto.ca. 5 mail.mumble.utoronto.ca. ; sdig a mail.mumble.utoronto.ca. 128.100.X.Y ; sdig a jackson.mumble.utoronto.ca. 128.100.X.Y
(mumble is not the real subdomain name; I just decline to identify them here because, well.)
So that's three MX records, two of which are literally redundant with each other, and all of them are pointing at the same machine. I'm not sure what happened here; perhaps the DNS zone file is organized such that it wasn't immediately obvious to people that they already had MX entries when they added more MX entries, or something.
(Or perhaps someone took the advice that one should have redundant MX entries a little bit too literally, similarly to what some people have done with NS entries.)
One of the interesting consequences of triply redundant non-redundant MX entries is that some mailers will probably take two or three times as long as usual to time out on delivery attempts should your mail server ever be down. Other mailers are smart enough to notice that everything is pointing to one IP address and only do one delivery attempt. And either way, it's probably doing odd things to mailer retry timers.