Like many places with a lively and long email history, we generate a number of different sorts of outgoing email. These sorts of email have different characteristics and thus different chances of being seen as spam email or bulk email by outside places.

So here's a rough list of the different sorts of email that our system winds up sending out:

• direct email from users; the least likely to be considered spam, since the users are typing it themselves.

• users forwarding email to outside places; often winds up forwarding spam that got sent to the user.

• bounces from when users forward their email to somewhere that refuses spam at SMTP time; potentially triggers 'trying to email too many bad addresses' logic at email providers.

  (Various providers at least claim to have such logic in their SMTP error messages.)

• autoreplies from users (vacation messages and so on); potentially bulk and may also trigger the same 'too many bad addresses' issue as bounces.

• user-run mailing lists with outside subscribers; potentially seen as bulk email, and may be spam if spammers hit the mailing list address.

• departmental mail shots of various sorts of information (eg, departmental newsletters); quite possibly seen as bulk email, but probably not as spam.

These different types of email are also of very different importance to our users, with the most crucial sort being direct email from users (bounces are probably the least important).

Since outside places may have much more violent reactions to some of these than others, sending all of them out from the same machines risks having all of your email to somewhere get blocked if that somewhere decides it doesn't like one type. Thus, it makes a great deal of sense to arrange things so that various different sorts of email are sent from different machines, or at least from different IP addresses; this way if some destination reacts badly to some sort of email, at least only some of your email gets blocked.

(You can also segregate on other characteristics of the mail, such as what your anti-spam system feels about it.)