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subscribers are not sent emails, and may be periodically removed from the system (or restored, if the reason is no longer valid.)
These reasons are grouped into three categories:
Access to the subscriber's email was denied.
The email failed authentication checks (DMARC, SPF, or relay permissions).
The delivery attempt timed out before the email could be delivered.
The subscriber's email account is disabled.
The subscriber's email does not exist.
The subscriber's domain is blocked.
The subscriber's email address is blocked.
The subscriber's email address permanently bounced (the address does not exist or is unreachable).
The subscriber's IP address is blocked.
The subscriber's IP address is undeliverable.
The subscriber's email is malformed.
The subscriber's email is on an ESP denylist.
The subscriber is deemed undeliverable for an unknown reason.
The subscriber's mailbox is out of storage.
The email contained a problematic URL.
The email provider, i.e. Gmail, acknowledges the message, but is temporarily refusing to send the message due to overall volume coming from the sender. Sender can mean a number of different things. For instance, it can mean that your specific newsletter is sending a high amount of volume. It can mean your domain is sending a high amount of volume. Or, in rare cases, it can mean that Buttondown itself as an overall system is sending too much volume. You do not need to change or retry any errors you see due to rate limiting. Buttondown will automatically handle that. However, if you see this error come up often, consider more regularly sending your email at a specific cadence, i.e. weekly.
The email was marked as spam.
SPF validation failed for the email.
The email encountered a temporary delivery issue and will be retried automatically.
The subscriber's email is unreachable.