Sorry, I didn't see this mail until now.
Paul Smith writes:
The 'easiest' thing to do is to accept the message if it's going to
take over 30 seconds to process, and generate a 'bounce' later if
necessary - but I don't like that because of faked sender addresses.
Tweak the timeout so 99% of the rejections are in the SMTP session and
less than 1% using bounces, and you'll be fine.
I've heard good things about delays of 36 and 46 seconds. To my mind,
both seem a little long, but people I know have had good experience
with those timeouts.