Folks - after years of using AuthSMTP.com we have just changed our service for download delivery to SendGrid.com - We wanted more control over the delivery methods and failures which still happen once in a while.
We hope with the new mail delivery service the amount of failed emails will decay and less people will miss their precious download codes.
I tested the interface of the new service and like it a lot. Tracking codes, well documented API functions, etc. If you are looking for a new service to deliver a lot of eMails per month - you might want to check our their free accounts first. What finally brought me over to them was the fact that support questions via eMail were answered extremely fast! I thought we are good at that, but SendGrid did just blew me away with a turnaround time of less than 50 minutes with 3 support requests in a row. That is quick!