What is standard retry timing (RFC 5321)?

RFC 5321 recommends that sending systems retry delivery for at least 4 to 5 days before abandoning messages that receive transient failure responses. This accommodates servers experiencing extended outages.

The RFC suggests initial retry after a delay (not immediate), with subsequent attempts at increasing intervals. Specific timing is left to implementation, but the multi day window ensures reasonable delivery opportunity.

Modern practice often shortens this window for marketing email. Promotional messages have time sensitivity that makes multi day retry windows impractical. Transactional messages warrant longer retry periods given their importance to recipients.