You should use a filesystem with dynamic Inode allocation. I use XFS - it is incredibel fast and a journaling filesystem. It is even NFS-proof, so you can export your vpopmail-home to multiple hosts.
IMHO you should not use ext, because hosting 1.500.000 emails, stored in Maildirs, you will not see a good performance.
I can confirm this to some extend, due to a loop in a antivirus product, i ended up filling a little over 100gb with 5kb mails (thats roughly 20.000.000.000 emails) in the postmaster@ account.
on the 1.13ghz p4 512mb Qmail didnt have any problems handeling the mails in que, and delivering them to the account, but sqwebmail timed out when trying to index / count them, also the ext3 file system took so long on a "ls" that i after a long time eventually gave up and killed the process, and rm -rf'ed the "new" dir.
the lesson is as Jens points out, if you are going to use maildirs, you should use a file system thats geared to handeling huge amounts of small files.
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