5.1.1 Recipient address rejected is incorrectly processed as Soft Bounce

[Alpha1]

Affected version

2.3.7

Bounce message notification from Protonmail:
550 5.1.1

Recipient address rejected: Address is not configured to receive emails

Status code 5.1.1 is a permanent failure: Bad destination mailbox address.
AWS SES also classifies this as a permanent failure, but XenForo classifies it as a Soft Bounce.

SMTP Status code definitions:

https://www.zerobounce.net/docs/email-list-validation/the-zb-guarantee#smtp-bounce-codes