On Mon, Mar 04, 2002 at 08:07:47AM +0100, Martin Husemann wrote:
> > (The same goes for async mounts on *BSD, and afaict, softupdates -- but
> > neither of these is the default.)
> 
> No, not for softupdates (if they work correctly, which they are said to
> do now at least in current).

http://cr.yp.to/qmail/faq/reliability.html#filesystems

"What types of filesystems are safe for mail?

Answer: qmail's queue, except for bounce message contents, is crashproof
on the BSD FFS and most of its variants.

Do not use async or softupdates filesystems. If you do, and if your
system crashes at the wrong moment, you will lose mail. Under Linux,
make sure that all mail-handling filesystems are mounted sync. The same
comments apply to many other popular MTAs. (However, some MTAs are
unreliable no matter what filesystem you use.)"

qmail (and possibly some other MTAs) relies on synchronous behavior of
rename(), whereas it is an asynchronous call with softupdates.