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 delive

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.