What you want in a auto responder is deliver ability, because you want you mails to
reach your prospects, and not get slam dunked in to the SPAM folder!
They also send out on-the-fly meaning once you press the "SEND" buttom your
mails are beeing delivered, other services it might take upto 24 hours.. which is
really bad if you`re doing a launch or need to send them an ASAP email! ;/