Why a good mail server refuses instead of bouncing
24 July 2026 · 4 min read
There are two moments a server can say no: during the SMTP conversation, or later, by generating a bounce. The first is free. The second sends a message to whatever address was in the envelope, which for spam is somebody who never wrote to you.
Backscatter
That is the name for those bounces. At volume it gets your server listed, and the victim is a third party who did nothing. Refusing during the conversation puts the decision back on the sending server, where it belongs.
What that means in practice
- Validate the recipient at RCPT time, not after DATA.
- Use 4xx when the problem is temporary, so real senders retry and nothing is lost.
- Use 5xx only when the answer will not change.
- Keep the wording identical across cases you do not want to distinguish.
That last point is a privacy property. If "no such address" and "this address is blocked" read differently, a spammer learns which addresses belong to real people, which is worth more to them than the address was.
More on deliverability
SPF, DKIM and DMARC in plain English
Three records, three different jobs, and one relationship between them that explains almost every "why is my mail in spam" question.
Why forwarded mail lands in spam
Forwarding breaks SPF every single time, by design. What keeps forwarded mail out of the spam folder is DKIM surviving the trip, and that is easy to break.
MTA-STS and TLS-RPT, and when they are worth it
SMTP encryption is opportunistic by default, which means it is trivially downgraded. These two records are the fix, and one of them is nearly free.