[kilburn]

I can not find a reference to explicit "DOs and DONOTs", but you can surely gather experience from systems that have used this schema for a looong long time: mail handling systems.

For a quick start, I would look at the maildir specification, that includes instructions on how you should read form and write to maildir folders to avoid locking and get good performance: http://www.qmail.org/man/man5/maildir.html

Then, I would dive deeper by looking at the processes used to maintain the mail queues in qmail: http://www.qmail.org/qmail-manual-html/misc/INTERNALS.html . Obviously, you could also look at how postfix or exim handle their own queues.

Anyway, gathering all the experience buried in those systems and summarizing it in a logical way would make a great great article...