SMTP policy requires retrying if mail cannot be delivered. From RFC 5321 section 4.5.4.1:
> In a typical system, the program that composes a message has some method for requesting immediate attention for a new piece of outgoing mail, while mail that cannot be transmitted immediately MUST be queued and periodically retried by the sender.
[ ... ]
> Retries continue until the message is transmitted or the sender gives up; the give-up time generally needs to be at least 4-5 days.
> In a typical system, the program that composes a message has some method for requesting immediate attention for a new piece of outgoing mail, while mail that cannot be transmitted immediately MUST be queued and periodically retried by the sender.
[ ... ]
> Retries continue until the message is transmitted or the sender gives up; the give-up time generally needs to be at least 4-5 days.