How can I interpret provider-specific bounce details?

You get a bounce back, and the message looks like someone typed it in a foreign language. Something like 550 5.1.1 The email account that you tried to reach does not exist or 421 4.7.0 Try again later, sending too quickly. What does any of that actually mean?

Here's how to break it down.

Start with the three-digit code

The first number tells you how bad it is. A 5xx code is a permanent failure (sometimes called a hard bounce). The receiving server is telling you clearly that this email is not going through, now or ever. A 4xx code is a temporary failure (soft bounce). The server wants you to try again later. A 3xx code is a redirect, which you rarely see in bounce messages but it does exist.

Now if you see a 5xx, suppress that address. If you see a 4xx, your sending platform will usually retry automatically, but if the same address keeps producing 4xx codes over days, treat it like a 5xx and pull it.

Read the enhanced status code

Right after the three-digit code, you'll often see something in X.Y.Z format, like 5.1.1 or 5.7.26. These enhanced status codes give you the category before you even read the human-readable text.

• 5.1.x usually means a problem with the address itself (unknown user, bad mailbox)
• 5.2.x usually means the mailbox is full or over quota
• 5.4.x usually means a routing or network issue
• 5.7.x usually means a policy or reputation rejection

Now the 5.7.x family is the one to watch closely. It covers authentication failures, spam policy rejections, and blocklist-based refusals. That's where most deliverability problems live.

Then read what each provider adds on top

Each major mailbox provider layers their own language on top of the standard codes. That's where things get specific.

Gmail tends to be descriptive. A message like 550 5.7.26 This message does not pass authentication checks tells you exactly what's wrong. Gmail also publishes reason codes in its Postmaster Tools, which give you even more granular signal. Check support.google.com/mail postmaster articles when you see a Gmail-specific string you don't recognize.

Outlook and Microsoft 365 use alphanumeric error codes like OU-001, BL000001, or structured codes like 5.7.1 with a Microsoft-specific explanation appended. The docs.microsoft.com Exchange error reference pages are your best friend here. Many Microsoft rejections include a URL in the bounce body pointing directly to the relevant policy page.

Yahoo Mail uses phrases like "deferred due to policy" or "Message from IP temporarily deferred" for soft blocks, and explicit codes like 554 Message not allowed for hard rejections. postmaster.yahooinc.com covers their published codes, though Yahoo is notoriously stingy with details on policy-based deferrals.

A practical workflow

1. Capture the full SMTP response from your ESP's bounce log, not just the summary your platform shows in its dashboard. Dashboards often strip the useful detail.
2. Search the exact error string plus the word "Postmaster" (for example: Outlook OU-002 Postmaster). Provider support pages usually surface quickly.
3. Classify the bounce. Is it about the address (5.1.x), the mailbox (5.2.x), or your reputation (5.7.x)? That tells you whether to fix your list or fix your sending.
4. Note repeating patterns. If you're seeing the same 5.7.x code from the same provider across dozens of addresses, that's a reputation signal, not a list quality problem.

Over time, the patterns become familiar. You'll start recognizing Microsoft's policy pages on sight, or knowing that a Yahoo deferral after a certain volume usually clears in 24 hours. That experience only comes from reading the full bounce, every time, not just the headline code your ESP shows you.

If your bounce rates are climbing and you're not sure whether it's a list problem or a reputation problem, our free blocklist checker is a good first stop. Or if you're staring at a bounce string that genuinely makes no sense, drop us a line through the SOS hotline and we'll help you decode it.