== Don't support shortened versions of your domain names in email addresses

I'll start with the basic background: the university here is divided
up into a lot of departments and divisions and groups and so on.
Most of them have subdomains under our top level domain, for instance
_cs.utoronto.ca_ (and then hosts under their subdomain), and many of
them run (or ran) their own email systems, creating email addresses
like _<user>@cs.toronto.edu_.

[[We http://www.cs.toronto.edu/]] have been providing email to
people for what is now quite a long time. Way back at the beginning
of that time, so long ago that DNS itself was very shiny and new,
we decided to let people send email using shortened versions of all
of the university's subdomains. Rather than having to send email
to '_<user>@ece.toronto.edu_', you could send it to just '_<user>@ece_'
and the mail system would work out where it should really go.  This
saved a bunch of people a bunch of typing back in the days when you
had to type all of the email addresses out and was much appreciated.

Of course people got used to using these short domain names. Of
course people put them everywhere, because they were generally
supported; you could put them in _.forward_ files, you could put
them in [[simple mailing lists SimpleEximMailingLists]], and so on.
And as a result of this, of course we've never depreciated this
feature.

If you guessed that supporting this feature in modern MTAs is a
slowly increasing amount of work, you're correct; it absolutely is.
If you guessed that it kind of goes wrong sometimes, you're also
correct (I discovered a case today). So I have a simple suggestion:
if you're tempted to do this because it sounds like a neat convenience
feature, *don't*. Forcing everyone to use fully canonicalized domain
names will make your life much simpler and will avoid a certain
amount of pain.

(It's probably going to get worse, since DNS keeps growing more and
more weird top level domains. Sooner or later we're going to have a
serious collision between a new one and one of our own subdomains.)

The hard thing about this is that I don't think we were wrong to
support shortened versions of the university domains way back when, and
depreciating them at any time since then would undoubtedly have been a
lot of work and not have been appreciated at all. But I still think that
in the modern world it would be better if we weren't supporting them any
more. In a world with MUAs that have address autocompletion and so on, I
think they're not necessary and they do cause problems.

(When I say that 'we' did this way back when, I mean the people who
were sysadmins here in those days, not me personally.)

=== Sidebar: one way to have a collision heartburn

Suppose that you have a university subdomain, let's call it _aa_
for illustration; this subdomain has a wildcard _A_ record for its
own reasons. There's also a _aa_ TLD (here it would be a country),
and one of your users tries to send email to '_<user>@misspelled.aa_'.
With the best of intentions, your MTA is almost certainly going to
wind up trying to deliver this email to the _aa_ subdomain wildcard
_A_ record instead of rejecting it as 'no such domain exists'.

(If the target of the _A_ record doesn't respond to SMTP connections,
your MTA will be retrying it for some time (especially if you've
configured high timeouts for in-university addresses).)