Depends on how many accounts you have. Having a /var/spool/mail with 30,000 entries will definitely hurt performance. It also depends on how many boxes would be accessing the mail spool (locking issue). Also depends on what kind of mail volume you move.
One of the main advantages of the filer is that it supports directory hashing to speeds lookups greatly. This has been a feature since 2.1; there's a white paper on the web site that talks about it. You are definitely better off having 30,000 files in one directory on a filer than having it on a local UFS disk...
Some rules of thumb to follow:
1. Configure your clients so all email is delivered from one central mailhost (which then writes to the mail spool over NFS). The other clients will read from it and re-write their own mailboxes, but delivery by the local agent (usually /bin/mail) is best left to only one machine.
2. Mount /var/spool/mail with the "hard" option... this prevents timeouts in case of overload.
3. Mount /var/spool/mail with the "noac" option... this is not necessary, but it is recommended. In fact, if you don''t do this, Solaris will execute the following code at startup:
/usr/bin/logger -p mail.crit "WARNING: /var/mail is NFS-mounted without setting actimeo=0," /usr/bin/logger -p mail.crit "this can cause mailbox locking and access problems."
4. Upgrade to the latest version of sendmail... vendor-supplied sendmails are often extremely buggy.
Bruce