norewrite (...) I take it this tells fetchmail not to write a intermediary file?
No. It has nothing to do with temp file usage. From the fetchmail manual:
-n, --norewrite
(Keyword: no rewrite) Normally, fetchmail edits
RFC-822 address headers (To, From, Cc, Bcc, and
Reply-To) in fetched mail so that any mail IDs
local to the server are expanded to full addresses
(@ and the mailserver hostname are appended). This
enables replies on the client to get addressed corÂ
rectly (otherwise your mailer might think they
should be addressed to local users on the client
machine!). This option disables the rewrite.
(This option is provided to pacify people who are
paranoid about having an MTA edit mail headers and
want to know they can prevent it, but it is generÂ
ally not a good idea to actually turn off rewrite.)
When using ETRN, the rewrite option is ineffective.
mda "/usr/bin/procmail -f %F"
This tells fetchmail not to deliver mail to <mailhost> Port 25. It transfers email directly to procmail. Therefore no /var/spool/mail/<username> is needed.