==================================== The mail logs show that these emails are typically delayed for up to 24 hours, sometimes longer. It appears that several attempts to deliver the email are made, with each unsuccessful one showing "Deferred" status in the maillog. After several hours of failed attempts the mail finally goes through successfully. Here is a typical entry (customer name redacted): exmx1 : Dec 14 05:28:39 exmx1 sendmail[31574]: mBDIYhEa031574: to=<customer at example.com>, delay=00:00:00, xde lay=00:00:00, mailer=esmtp, pri=41874, relay=mail.example.com., dsn=4.0.0, stat=Deferred Unfortunately there is no reason given for the "Deferred" status such as "Server unavailable", etc. ==================================== I would imagine this is a hotmail or yahoo account or something along those lines. Deferred does not usually mean bounced or turned into spam, it is more of a way of making the user wait for their mail. Yahoo can hold mail for a long time, minutes to hours, before they deliver. If you keep resending the deferred mail, you may end up getting auto junked though. In my minds eye, they are just trying to get people to upgrade as then as if by magic, mail to them does not get deferred. But in my experience, all deferred mail is usually sent, as far as I know. Just 'deferred' for a bit.