[PatrolX]

Deliverability is a non-issue if you hook tools like this and Sendy up to Amazon SES with DKIM and verified MAIL FROM settings. I've been doing it for years and never once had a deliverability issue. I've also sent directly from EC2 instances without issues but SES is so cheap and reliable I've stuck with it.

[webbie917]

Sounds like you have a good and established sender reputation, which is a big factor. When people start fresh (new sender identity) they tend to run into deliverability issues.

Also, the quality and size of the starting list matters too because a large list (like the OPs) or unengaged list can have a high bounce and low engagement rate, which factors into future deliverability.