We've been using the following auto_direct entry for mounting /var/mail on Solaris 2.6, Solaris 8, and <cough> SunOS 4.1.3 clients for years without incident:

/var/mail -rw,intr,hard,actimeo=0 toaster:/vol/fs0/mail_spool

However, we don't use the standard Sun Sendmail configs. either.