Not really employment but need help with something in work

[poorbuthappy]

Sorry if there is another to post this in...

Working on companies terms and conditions of sale, completely revised and updated and we are wondering about email as a "official" way of communication.
Due to the industry almost everything we do sales wise is via email and mostly no issues, well actually it works very well!

But alongwith putting email in as line of communication between us and customers, we also have to mark out the boundaries as to when the communication is deemed received:

so, 1st class post, X number of days later
delivered by hand - on day of delivery
fax - before 4pm same day, after 4pm next day.

When can we deem an email delivered?

(just to say due to email traffic I really don't want to put a load of sales engineers on read receipts, esp knowing that many people don't actually click yes when asked. Personally if very very important I make a phone call first (if poss) and talk about the email, then send it as confirmation of conversation and ask for confirmation of receipt by return email - but this seems to be overkill on standard emails!)

Any ideas anyone?