Version 7 mailboxes have several issues, esp when they get large.
Corruption is not one of them.  I'm not going to debate it.
I've worked for a very mail-intensive company for the last 4 years,
and been dealing with SMTP mail on unix for the 6 years before
that for excessively large corps and ISPs.  Corruption happens
with multiple writers and poor locking.

That said, I dislike "mbox" format (properly Version 7 mailspools)
for several uses.  With IMAP (UW) and POP (UW/Qpopper) deleting
a message in the middle of a large box means ugly rewrites and
hammering IO.

Courier has some arguable IMAP issues (it implements what the author
seems to think rfc2060 should say, rather than what it actually says).
Problems are fairly rare.  It's a young product.

Cyrus and Cyrus based IMAP servers perform well.  Cyrus doesn't
require Unix accounts for auth, it can use a festival of external
auth methods as well as an internal one.  It uses a file/message.
Delete message 200 from a 20k mailbox and it's a simple unlink.

For a variety of reasons, I've been using Sendmail Inc's IMAP
server which is based on Cyrus with a pretty GUI in front of
it (Mom can manage adding/removing users to her company via
a web GUI).  Oh, one message to 50 people becomes 1 file with
49 additional links - useful with sales people and huge PPT files.
Useful with students emailing mp3s and the like to their pals.
Reduces lots of IO.  Kinda of odd to see 1200 GB in use on an
800GB system that still has room, though.

IMP, Squirrel are two fine free Webmail products.  Not sure they'd
scale to 50k users.

Quoting Eric Gillespie (epg@pretzelnet.org):
> Richard Rauch <rauch@rice.edu> writes:
> 
> > (a) As I read about postfix, I see that I have two (at least) options for
> >     local mail delivery.  One is "Maildir/", which apparently breaks each
> >     message into a seperate file.  The other is the conventional UNIX
> >     single-file mailbox.
> 
> Unless you have to support users who *need* mbox, go with
> maildir.  Delivery to a maildir is atomic, while mbox is
> guaranteed to give you a corrupt mailbox once in a while.
> 
> >     Any recommended POP3 or IMAP servers?  Or, for that matter, web-mail
> >     servers that I might want to try?  (^&
> 
> I use Courier-IMAP and Sqwebmail, both over SSL.  I see a lot of
> recommendations for Squirrelmail, but it has a lot of overhead
> (PHP: just what you need, Yet Another Security Hole).  Sqwebmail
> has a simple, secure design (not saying Squirrelmail doesn't, i
> can't comment there), is written in C, and runs as a CGI.  Very
> fast and reliable.