So I talked to both our engineers and the eng team from our integrations partner. Alan T. & Westpoint C.
the “recipient rejected” error happens on the receiving end after the email is successfully sent. usually it’s their spam filters or domain reputation issues with bulk outreach.
quick fixes to try:
warm up your domain with smaller volumes first
personalize content to avoid looking automated
check your SPF/DKIM records
use a separate sending domain
this isn’t something we can fix on our end since it happens after the email leaves our system