Welp! Time to mark as resolved, because I fixed it!
TL;DR There was some outdated stuff, including users, in sentora_postfix DB. Sorted it out by getting the proper postfix user in with the right password set by Sentora via the install.
TL;DR There was some outdated stuff, including users, in sentora_postfix DB. Sorted it out by getting the proper postfix user in with the right password set by Sentora via the install.