SMTP requires the sender to retry delivery if the destination is unreachable.<p><a href="https:&#x2F;&#x2F;tools.ietf.org&#x2F;html&#x2F;rfc5321#section-4.5.4.1" rel="nofollow">https:&#x2F;&#x2F;tools.ietf.org&#x2F;html&#x2F;rfc5321#section-4.5.4.1</a>