autovacuum

I have a large database used with our mail filter. The pg_dumpall
results in about 3GB with this being the only database in the system
besides templates and postgres.

I do a vacuum every night after backup and it takes about an hour, is
this normal for this size db? I also have autovacuum enabled and when it
is running during the day, our mail queues will tend to fill up with
slow response to the server. Should I have autovacuum on even if I am
vacuuming the db every night?

Let me know if you need more specifics. Just trying to get some feedback
on if my vacuum is taking too long or if both are necessary...thanks for
the help!

--
Robert