Hotmail, the email incinerator
A heads-up for anyone who operates any sort of system which sends emails to customers - Hotmail could well be rejecting your email and not telling you, or the recipient, that they've done so. You can get yourself whitelisted by signing up to Bonded Sender, but that costs money. Oh, and the threshold for Bonded Sender to reject your application is 1 complaint per one million emails sent. Oh, and if they reject your application you can't reapply for 90 days.
The full skinny can be found on e-consultancy . If you're a large organisation sending out a lot of email you might like to reassure yourself that a significant proportion of your customer base doesn't think you're ignoring them.