"If the mailserver we're sending to can tell immediately that it's not going to deliver the mail it can respond with a 5xx or 4xx response during the SMTP transaction - a rejection (aka a synchronous bounce) or a deferral."
"One big problem with asynchronous bounces is that it's pretty common for spammers to lie and put the email addresses of unrelated third-parties in the return path, meaning that the asynchronous bounces are sent to those people."
"Backscatter looks pretty much like spam to those who receive it, and so could lead to the mailbox provider that's sending those bounces getting blocked."
"Best practice for decades has been to minimize backscatter to the lowest levels possible, by avoiding sending asynchronous bounces wherever reasonable."
Email delivery failures can be communicated through synchronous bounces during the SMTP transaction or asynchronous bounces sent later. Asynchronous bounces often result in backscatter, where unrelated third-party email addresses receive bounce messages due to spammers manipulating return paths. This creates issues for mailbox providers, as excessive backscatter can lead to being blocked and is considered poor engineering practice. Best practices recommend minimizing backscatter by making delivery decisions during the SMTP connection to avoid unnecessary bounces.
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