Like Mandrill, most (all?) of these companies have some warmup period. Start sending low volumes, then once you have enough reputation go to higher volumes. I

Like Mandrill, most (all?) of these companies have some warmup period. Start sending low volumes, then once you have enough reputation go to higher volumes. I don't belong to any of these companies but as a client, use their services heavily. Imagine a spammer signing up with them and sending several thousands mails in a day. They deal with such notorious people on a daily basis. I am not saying you may have done anything wrong but probably their system flagged you when you sent 11k emails to 10-15 people. In any case here is my rec for you in order- 1. PostMark - They do strictly transactional email. 2. Mailgun 3. SparkPost 4. SES