What are SMTP reply codes?

You send an email. Somewhere between your outbox and the recipient's inbox, a server responds with a three-digit number. That number is an SMTP reply code, and it tells your sending server exactly what happened and what to do next.

Every reply code follows the same structure. The first digit is the most important one. It tells you whether the action succeeded, failed temporarily, or failed permanently. The second digit narrows it down to a category (like the network layer or the mailbox itself). The third digit gives you the specific condition.

Here's how the first digit breaks down:

• 2xx means success. The server accepted the command and everything went fine. A 250 OK after DATA is the one you want to see.
• 3xx means "keep going". The server is waiting for more input before it can complete the action.
• 4xx means a temporary failure. The server had a problem right now, but it might work if you try again later. Your sending server should retry automatically.
• 5xx means a permanent failure. Don't retry. The message was rejected and it's not coming back.

A few codes you'll see often: 220 is the server greeting ("I'm ready"). 250 is "that worked". 421 means the service is temporarily unavailable, so try again. 550 usually means the recipient address doesn't exist or your message was refused outright.

Why does this matter for you? Because when a campaign bounce report says "550 5.1.1" or "421 Too many connections", those aren't random error messages. They're specific signals about what went wrong. A 4xx tells you to keep the address on your list and wait. A 5xx tells you to suppress that address immediately.

If you're seeing a lot of 5xx codes and aren't sure what's driving them, our SOS hotline is free and we'll help you read what the servers are actually saying.