For high-latency links, such as heavily contended dialup access,
fetchmail has very poor performance since it has to do several
round-trips for each message it fetches.
It's much more efficient to batch up all the messages and send them in one
stream, then process them on the client (both because this avoids round-trips
and because it permits better compression); however, fetchmail currently has no
way of reading messages from a file or pipe rather than via one of the mail
retrieval protocols it understands, so I can't use it to inject mail into my
local MTA.
I'd like fetchmail to have a --from-file option which reads from a
local mbox file so that I can sync my mail across, then use fetchmail as I
normally would. (Currently I'm doing formail -s procmail on the
downloaded mailbox, which works, but avoids the normal processing that my local
MTA does.)