Automated Email From Websites to Customers (Jakob Nielsen's Alertbox)
This one hit close to home. Though the article is about automated transactional emails, it contains some good nuggets that I think are important for automated email from an online community.
Importance of the "From" field and "Subject" line. I some platforms, like Web Crossing you can easily manage the from field to say discussions@commoncraft.com (or whatever you want). This is important for helping the users understand the content of the message as quickly as possible.
For the subject line, I like to use a subject line ID like [Common Craft] so that the user immediately knows where the message is from. Also, this allows filtering in the email program.
The articles also showed that users want the most important information at the top- no messing around with fluff.
In terms of email digests of online community messages, I think they should contain a table of contents that separates new discussions from replies and provides links to specific messages in the email digest from the table of contents.