What are typical Gmail vs Microsoft error code patterns?

You get a bounce back, and now you're staring at something like 550-5.7.26 This message does not have authentication information from Gmail or 421 RP-001 from Outlook. They're both rejections, but they read completely differently. That's because each provider has its own way of packaging the bad news.

Here's how to decode what each one is actually telling you.

Gmail error patterns

Gmail wraps standard 5xx SMTP codes with long, human-readable text. That verbose text is a feature, not noise. It usually tells you exactly what went wrong and sometimes even hands you a link to the fix.

• 550-5.1.1. The recipient address doesn't exist. Hard bounce, remove it.
• 550-5.7.26. Failed authentication. Your SPF, DKIM, or DMARC isn't passing. This one is a policy block, not a reputation block.
• 550-5.7.1. Message rejected because of sender reputation or policy. Could be a blocklist hit or a content issue.
• 421-4.7.0. A temporary rate limit or throttle. Gmail is asking you to slow down, not stop permanently.

The rule of thumb with Gmail: read the whole error string, not just the numeric code. The text after the dash usually tells you whether it's an authentication issue, a reputation issue, or a bad address. When Gmail includes a support.google.com URL in the bounce, follow it. They actually document their error codes well.

Microsoft (Outlook and Microsoft 365) error patterns

Microsoft 365 and Outlook use alphanumeric sub-codes layered on top of the standard SMTP numbers. If Gmail is verbose, Microsoft is terse. You'll see things like 421 RP-001 or 550 5.7.1 Service unavailable; Client host blocked. The alphanumeric tag is the key piece.

• 421 RP-001 / RP-002. Rate limiting. You're sending too fast to Microsoft's servers. Back off and retry.
• 550 5.7.1 with SC-001. Your IP or domain is on a Microsoft blocklist. You'll need to use their Smart Network Data Services (SNDS) or submit a delisting request.
• 550 5.1.0. Sender denied, often tied to poor reputation or a policy filter at the tenant level.
• 550 5.2.2. Recipient mailbox is full. Soft bounce initially, but if it keeps happening it's worth suppressing.
• 550 5.7.606. Sending from a new or suspicious IP. Microsoft wants to see a warm-up history before trusting you.

The alphanumeric codes (RP-001, SC-001, etc.) are Microsoft's internal routing tags. They're not always documented publicly, but Microsoft's postmaster pages cover the most common ones. If you hit something unusual, search the exact code string including the tag.

Yahoo error patterns

Yahoo Mail uses a different flavour again. Their bounce messages often reference complaint rates or policy violations, and they have their own internal codes for throttling.

• TS01 / TS02 / TS03. Throttling codes. Yahoo is deferring your mail, usually because your sending volume spiked or your complaint rate is elevated.
• 554 Message not allowed. Policy rejection, often tied to content or reputation.
• 421 Resources temporarily not available. A soft deferral. Retry with backoff.

Yahoo's error messages tend to be shorter on explanation than Gmail's. When you hit a Yahoo policy block, the first thing to check is your complaint rate in Yahoo Postmaster Tools. That's usually what's driving it.

How to actually use this

Now the practical workflow is the same regardless of provider. First, check whether it's a 4xx (temporary, retry allowed) or a 5xx (permanent, fix required). Then read the sub-code or tag to understand whether it's an address issue, an authentication issue, a reputation issue, or a rate issue. Each of those has a different fix path.

Authentication failures (like Gmail's 5.7.26) need you to check your SPF and DKIM records. Reputation blocks (Microsoft's SC-001) need a delisting request plus underlying list hygiene. Rate limits (421 codes from anyone) just need a sending slowdown. Don't treat them all as the same problem.

If you're seeing a pattern of bounces and aren't sure whether it's your domain, your IP, or your content, our free Email Header Analyzer can help you see what the receiving server actually decided. Or if it's getting urgent, our SOS hotline is free and we'll help you work through it.