What are some known specific bounce messages from Outlook?

You sent a campaign, a chunk of it bounced, and the error messages look like someone sneezed on a keyboard. 550 5.7.606? 421 RP-001? Outlook and Microsoft 365 use a structured bounce code system that actually tells you a lot once you know how to read it.

The first number tells you the category. 421 means a temporary rejection (soft bounce, try again later). 550 means a permanent rejection (hard bounce, don't retry). The sub-code after it, like 5.1.1 or 5.7.606, narrows down the exact reason.

User and mailbox issues (550 5.1.x and 5.2.x)

• 550 5.1.1 User unknown. The address doesn't exist. Remove it.
• 550 5.1.10 Recipient not found. Similar story, the mailbox isn't there. Remove it.
• 550 5.2.1 Mailbox disabled. The account exists but has been deactivated. Remove it.
• 550 5.2.2 Mailbox full. Technically a soft bounce, but if it persists across multiple sends, it's a dead address. Move it to your monitor or suppress list.
• 550 5.3.4 Message size exceeds limits. Your email is too large. Trim attachments or images.

Rate limiting (421 RP-series)

• 421 RP-001, 421 RP-002, 421 RP-003. Microsoft is throttling your sending. These are temporary. Slow down your send rate and let the queue retry. If they keep appearing, your sending reputation needs attention.
• 421 4.7.0 Connection rate limit exceeded. Same idea. You're connecting too fast. Back off and retry.

Policy and reputation blocks (550 5.7.x)

• 550 5.7.1 Service unavailable, client host blocked. Your sending IP or domain is on a blocklist or has tripped Microsoft's filters. This one needs investigation, not just a retry.
• 550 5.7.606 Access denied, banned sending IP. Your IP has been explicitly banned. This is serious. You'll likely need to switch IPs and work through Microsoft's Sender Support portal for remediation.
• 550 5.7.708 Access denied, traffic not accepted from this IP. A similar block, often applied to IP ranges with poor reputation history.

The RP-series and 5.7.xxx codes are Microsoft's fingerprints. If you see them, you're dealing with Outlook or Microsoft 365. The references to sender.office.com in rejection messages are another giveaway.

Now the main thing to take away: 5.1.x and 5.2.1 codes mean bad addresses, so clean those up immediately. 5.7.x codes mean Microsoft has a problem with your reputation or IP, and retrying harder won't fix that. You need to understand what each bounce type actually requires from you before hitting send again.

If you're seeing a lot of these at once, it might be worth running your domain through our free blocklist checker to see if something bigger is going on. And if the 5.7.606 or 5.7.708 codes are showing up regularly, that's a conversation worth having with a real human. Our SOS hotline is free.