How do Gmail, Yahoo, and Outlook label bounces differently?

You pull a bounce report and suddenly you're reading three different languages. A Gmail rejection looks nothing like an Outlook one, and Yahoo does its own thing entirely. The good news is each provider follows a consistent internal logic, so once you know the pattern, you can decode any bounce message they throw at you.

Before diving in, a quick grounding point. All three providers build on the same base: SMTP status codes in the 4xx (temporary, retry) and 5xx (permanent, don't retry) range. What varies is the layer of extra information each provider wraps around those codes.

Gmail: wordy by design

Gmail writes bounce messages like it's explaining itself to a human. You'll get multi-line responses with full sentences, often including a link to a Google support page. That verbosity is actually useful. Gmail tells you exactly what triggered the rejection.

Common patterns to know:

• Spam policy: "Our system has detected that this message is likely unsolicited mail."
• Authentication failures: Gmail will call out SPF or DKIM issues explicitly in the bounce text.
• User unknown: "The email account that you tried to reach does not exist." Clean and unambiguous.

When Gmail rejects you, read the full message. The explanation is usually right there. Then check the linked support doc if one appears, because Google often updates its policy pages with current guidance.

Yahoo: codes first, context later

Yahoo leans on short codes rather than prose. The most famous is the TS series. A "421 4.7.0 [TS01]" means your messages are being deferred because of a policy decision, usually tied to sending reputation or volume. TS02 indicates a different throttling trigger. You won't always get a detailed explanation, which is why Yahoo bounces feel more cryptic than Gmail ones.

Key things to watch for with Yahoo:

• 4xx TS codes are deferrals, not permanent failures. Yahoo is telling you to slow down or wait, not to give up.
• 5xx rejections from Yahoo are harder blocks, often reputation-based. These need investigation before retrying.
• Yahoo's 'deferred due to policy' messages have their own quirks worth understanding in detail.

Outlook and Microsoft: structured codes, terse text

Microsoft uses a two-layer system. You get the standard SMTP code (like 550), followed by an enhanced status code (like 5.7.606), sometimes followed by an alphanumeric reference code (like RP-001 or SC-001). The text message itself is usually short. Most of the meaning lives in those codes.

But some common ones:

• 550 5.7.606: Banned sending IP. Your IP is on a Microsoft blocklist.
• 550 5.7.1: The message was rejected due to content or policy. Could be reputation, could be authentication.
• 550 5.1.10: The recipient doesn't exist. Hard bounce, suppress it immediately.

Microsoft's alphanumeric codes (the RP, SC, BL prefixes) map to specific blocking reasons in their Microsoft 365 documentation. When you hit one, look it up. The code tells you which blocklist or policy rule fired, and that usually tells you what to fix.

How to use this in practice

So when a bounce lands, the first question is always: is this a 4xx or 5xx? Temporary or permanent? Then look at which provider sent it and match the format to the right reading pattern above. Gmail gives you plain language. Yahoo gives you a TS code. Microsoft gives you a structured code chain.

If you're seeing a lot of bounces across all three, your sending reputation is probably the common thread, not a provider-specific quirk. That's worth a deeper look at your full bounce picture before chasing individual codes.

Need help reading a specific bounce message? Paste your email headers into our free Email Header Analyzer and see what's actually going on under the hood.