Buttondown Documentation
The reason Buttondown has marked the subscriber as undeliverable. Undeliverable subscribers are not sent emails, and may be periodically removed from the system (or restored, if the reason is no longer valid.)
The subscriber is deemed undeliverable for an unknown reason.
The subscriber's IP address is blocked.
The subscriber's domain is blocked.
The subscriber's email address is blocked.
The subscriber's mailbox is out of storage.
The subscriber's email account is disabled.
The subscriber's email is unreachable.
Access to the subscriber's email was denied.
The subscriber's email does not exist.
Delivery to the subscriber's email was rate limited.
The email was marked as spam.
The email contained a problematic URL.
The subscriber's email is on an ESP denylist.
SPF validation failed for the email.