Not OP but I've heard (anecdotally here and there) that many sites reject email from Amazon IP blocks because of historical spam problems.
I only encountered it personally a handful of times when using SES for delivery, and it was usually resolvable because the target customer base was easy to reach and worked with their IT departments to solve the problem.
I only encountered it personally a handful of times when using SES for delivery, and it was usually resolvable because the target customer base was easy to reach and worked with their IT departments to solve the problem.