Hello Coolg,
Receiving email servers can have any number of rules for incoming emails and can reject them at any time. That said, there are a couple of things you can do to give your specific emails more credibility and be less likely to get rejected. I am including a link here on how to set up SPF records and domain keys for your account. This helps authenticate your emails for the receiving servers.
If you do this and still get rejected, there could be other reasons for the rejection. In those cases, simply send a copy of the bounceback message to our support team at [email protected] along with standard verification (last 4 digits of the credit card on file or current AMP password) and they can help identify and fix the issue.
I hope this answers your question. If you have any more questions or comments on the issue please leave a comment below so we can assist you further.
Thank you,
Scott M