How does Microsoft classify errors (5.1.0, 5.2.2, 5.7.1)?

Your email just bounced back from a Microsoft address with a code like 5.7.1 or 5.2.2 and you're trying to figure out what it actually means. Good news: Microsoft's error codes follow a consistent pattern once you know how to read them.

Every code has three parts. The first digit is always 5 for a permanent failure (as opposed to a 4, which means temporary and worth retrying). The second digit tells you the category. The third digit (and any extra digits) gets more specific about what went wrong.

Here's what each category covers:

5.1.x. Address problems

• 5.1.0: Generic address failure. Something is wrong with the recipient address but Microsoft isn't giving you a more specific reason.
• 5.1.1: The destination mailbox doesn't exist. This is your classic hard bounce. Remove this address from your list.
• 5.1.10: Recipient not found. Similar to 5.1.1 but often seen when the domain exists and the mailbox simply doesn't.

5.2.x. Mailbox problems

• 5.2.1: The mailbox exists but it's disabled or not accepting messages right now.
• 5.2.2: The mailbox is full. This is a soft bounce situation. You can retry, but if it stays full, eventually you should suppress it.
• 5.2.121: You've hit the recipient's per-hour message limit. You're sending too many emails to this one address in a short window.

5.7.x. Security and permissions

• 5.7.1: Delivery not authorized. This is Microsoft's catch-all block code. It can mean your IP is blocked, your domain has a bad reputation, or your authentication is failing.
• 5.7.509: DMARC validation failure. Your message failed the DMARC check at Outlook. Check your SPF, DKIM, and DMARC alignment before resending anything.
• 5.7.606: Your sending IP is on a banned list. You'll need to go through Microsoft's delisting process.
• 5.7.708: Microsoft isn't accepting traffic from your sending source at all. This is more severe than 606 and often means a reputation problem at the IP or network level.

If you're seeing 5.7.x codes in volume, that's the most urgent category. Address problems (5.1.x) usually just mean a bad list. Mailbox problems (5.2.x) are often temporary. But security blocks from Microsoft can affect your entire sending stream if you don't catch them early.

Now if 5.7.509 is showing up, run your domain through our free DMARC parser to check what's in your record, and follow up with the SPF checker to make sure your sending domains are aligned. If you're stuck on a 5.7.606 or 5.7.708 and need help working through a Microsoft delisting, the SOS hotline is free.