SMTP servers queue messages and retry periodically. It’s an old and resilient protocol. Eventually the message will bounce, but it’s up to the sending SMTP serv

SMTP servers queue messages and retry periodically. It’s an old and resilient protocol. Eventually the message will bounce, but it’s up to the sending SMTP server how long to retry before giving up. There are some guidelines here: https://datatracker.ietf.org/doc/html/rfc5321#section-4.5.4 https://datatracker.ietf.org/doc/html/rfc5321#section-4.5.4