The single feature from the original commercial version of
MailManager that has not been implemented in the new open
source version is handling confirm receipt requests.
See RFC 2298 http://www.faqs.org/rfcs/rfc2298.html
Ideally, the ticket screen should have an addition along the lines of
"Read confirmation requested by sender: click here to send a
confirmation" if a read confirmation has been requested.
Possibly there should also be an option to request a receipt
confirmation when sending mail too.