SDF receives mail for user from remote sender
SDF's local mail delivery agent (MDA) consults $HOME/.forward
if .forward calls procmail, then the MDA pipes the mail to procmail for filtering, with $HOME/.procmailrc
Procmail runs on the mail machine for which $MAILDIR (/mail/maildir/…) is local (not across the NFS (network file system) wire.
Best practice is for procmail filtering destinations to be local to where procmail is running, in $MAILDIR.
Unfiltered mail is then returned by procmail to the MDA which deposits it in $MAIL, also local to the mail machine, and is the users INBOX
User clients (i.e. mail, mutt, alpine, etc.) may read mail from $MAIL, and from $MAILDIR files, across the NFS wire to the $HOME machine for reading.
The local client may then store the mail locally in $HOME or back on the mail machine in $MAILDIR