Kinda surprising, but it DOES have something to do with Cyrus. Caspur did their case study on cluster filesystems with their e-mail environment. It used Cyrus IMAP and some kind of SMTP (I think it was Postfix or
Their paper talks about Maildir. If you connect to mailbox.caspur.it:993 you'll see a Courier-IMAP greeting.
Maildir is specifically designed not to need anything more fine-grained than filesystem metadata-level locking. In theory, it's even NFSv2-safe, though user-visible performance on typical IMAP loads sucks (see recent Thunderbird rants, and double). Cyrus usually expects more advanced locking facilities, but it depends on db backend.
I obviously can't argue with your own positive experiences with Tru64 clustering (as with other former DEC products, I've heard only good things about Tru64, except that it's been mismanaged and mismarketed into irrelevance), but those of us on crankier OSes need to worry about such things. Sendmail 8.12.5 release notes: "NOTE: Linux appears to have broken flock() again"; UW-IMAP FAQ http://www.washington.edu/imap/documentation/FAQ.html#6.11, etc.
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