How many retries are acceptable for transient bounces?

RFC 5321 does not specify exact retry counts but establishes that transient failures (4xx codes) should be retried for a reasonable period. Common practice is 3 to 5 retry attempts over 24 to 72 hours before converting to permanent failure.

Retry intervals should use increasing delays (exponential backoff). Starting with 15 to 30 minutes, doubling intervals between attempts prevents overwhelming temporarily struggling servers while ensuring timely delivery when issues resolve.

Excessive retries waste resources and may appear abusive. Too few retries abandon messages that could have delivered. Balance persistence with efficiency based on message importance and recipient expectations.