You should segregate different traffic to different mailing lists
In a comment on my entry about how people can always unsubscribe to things, billings wrote:
We actually run a couple mailing lists at work that are set up so you can't unsubscribe, because they are the official communications from the college to staff, students or faculty (each their own list). I argued that we should allow people to unsubscribe if they want, but university policy makes us have to maintain the list for stuff like emergency alerts and official messages from the Dean. [...]
It is my considered opinion that this is a bad idea. Why it's a bad idea is a direct consequence of how people can always unsubscribe and how these two examples are not like each other.
To be blunt, it's quite likely that a lot of people are going to be completely uninterested in official messages from the Dean. If they come out with any frequency, people are going to 'unsubscribe' from them in some way on their client. The more emergency messages resemble the Dean's official messages, the more likely that they too have been quietly 'unsubscribed' from (whether or not the user intended to do this). If you really want people to get emergency messages (and you probably do), this is a very bad thing.
What you want to do here is to differentiate the emergency messages
from the Dean's messages as much as possible. The more you
differentiate, the less likely that people will miss emergency
messages. Because of how people are generally going to do filtering,
you want the user-visible message headers to be as different as
possible, ie a different
From:, a different
Subject: (with no common prefix), and so on.
Since mail clients may at least potentially notice list-related
headers when assessing messages, you want to use a different actual
mailing list too.
This also applies to less drastic splits in purpose, of course. But don't split too finely, because then you make it too much work for people to unsubscribe from everything they don't want and they start using blunt hammers. To be honest there are only really three categories: lots of people are going to unsubscribe, you hope that only a few people unsubscribe, and you desperately want no one ever to unsubscribe. In an ideal world you'd have very few list splits beyond that (you might want to have a few different purpose-based splits in the middle category).