A thing that trips people up constantly, and the numbers behind it are worth having in one place. 𝟰𝘅𝘅 𝗶𝘀 𝗻𝗼𝘁 𝗮 𝗯𝗼𝘂𝗻𝗰𝗲. 𝗜𝘁 𝗶𝘀 𝗮 𝗱𝗲𝗹𝗮𝘆. The receiving server is saying try again later, and RFC 5321 section 4.2.1 is explicit that the client should. Mailbox full, greylisting, rate limit, transient DNS trouble. 𝟱𝘅𝘅 𝗶𝘀 𝗳𝗶𝗻𝗮𝗹. Same section: the client must not try again to the same recipient. So when a platform reports a soft bounce, nothing has failed yet. A queue has opened. How long that queue stays open is where it gets interesting, because it is a local decision and not a standard. 𝗗𝗼𝗰𝘂𝗺𝗲𝗻𝘁𝗲𝗱 𝗱𝗲𝗳𝗮𝘂𝗹𝘁𝘀:

• 𝗣𝗼𝘀𝘁𝗳𝗶𝘅: maximal_queue_lifetime defaults to 𝟱𝗱. Retry interval runs from minimal_backoff_time 300s to maximal_backoff_time 4000s.
• 𝗔𝗺𝗮𝘇𝗼𝗻 𝗦𝗘𝗦: 𝟭𝟰 𝗵𝗼𝘂𝗿𝘀 by default, configurable from 300 to 50,400 seconds since October 2024.
• 𝗚𝗺𝗮𝗶𝗹: no published retry window at all. That last one deserves a note. The 45 hours figure that circulates comes from DSN message bodies quoted in forums, not from any Google documentation. I went looking properly and it is not there. If someone has an actual documented source I would genuinely like to see it. The 5 day Postfix default is not arbitrary either. RFC 1123 section 5.3.1.1, which RFC 5321 inherits, says the give up time generally needs to be at least four to five days. Practical consequence: the same address gets an 8.5x difference in second chances depending on what is sending. If you suppress on first soft bounce you are throwing away deliverable addresses on one stack and not the other. What is your queue lifetime set to, and did you choose it or inherit the default?