I personally don't use any callouts for verifying recipient or sender since
the mail servers that I have handle huge amounts of users and email. Having
to do callouts to servers would drastically hit performance, and here's why:
1) slow mail servers could tie up callout verification if I don't have a low
timeout specified (and then I may never properly verify the address if the
server is constantly delayed)
2) if you have lots of email traffic, and you get hit with a spam/virus
attack with faked senders, you could end up doing tons of callouts, only
costing you more cpu time to deal with that before you even get to the
spam/virus scans
I'd say that if you have a low volume server, you could certainly try it
out. Specify a short callout period like 10s or something so you don't have
your server sitting around forever. You could even try without a callout
for a week, see what performance is like, then enable it and see what
difference it makes.
Personally the default check of testing the domain for an mx record is
enough for me - if there's at least a domain by that name that can accept
email, at least it should be possible to contact *someone* about problems.
Plus, sender verification must be done at RCPT or MAIL time, which is before
exiscan can do a spam/virus check, which means I must make Exim work as fast
as possible up until I can deny someone who is spamming (since I don't know
if they're a spammer at RCPT or MAIL time).
Hope that helps you with your final decision :)
Eli.
-----Original Message-----
From: exim-users-admin@??? [
mailto:exim-users-admin@exim.org] On Behalf
Of Phil Chambers
Sent: Monday, January 19, 2004 1:37 PM
To: exim-users@???
Subject: [Exim] verify = sender/callout
I am currently using verify = sender without the /callout option because the
manual
says "it can add a lot of resource usage to the cost of verifying an
address".
I am looking to see if most people have the callout switched on as it seems
to me
that it would screen out a significant proportion of spam, saving the cost
of
scanning.
Any advice?
Phil.
---------------------------------------
Phil Chambers (postmaster@???)
University of Exeter
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