What does 450 mean?

You fired off an email and got a 450 back. Before you panic, good news: a 450 is a temporary rejection. The receiving server is basically saying "not right now" rather than "not ever." Your message hasn't been lost. It just needs to wait.

The most common reason you'll see a 450 is greylisting. Some mail servers intentionally delay first-time senders by bouncing the initial attempt with a 450. When your server retries a few minutes later, the receiving server recognises you and lets it through. It's an anti-spam technique, and it works against spammers because most spam tools don't bother retrying.

Other causes include a mailbox that's temporarily locked (the user is doing something in their account), a quota issue on the recipient's end, or a policy check the receiving server hasn't finished running yet. None of these are your fault, and none of them require you to do anything dramatic.

What your retry strategy should look like:

• Wait at least 10 to 15 minutes before the first retry. Greylisting clears in that window for most servers.
• Use exponential backoff after that. Try again after 30 minutes, then an hour, then a few hours.
• Keep retrying for up to 48 to 72 hours. Most legitimate ESPs do this automatically.
• If you're still getting 450s after 72 hours, that's worth a closer look (see below).

Don't suppress the address on the first 450. That's a common mistake. A single temporary failure tells you nothing about whether the address is real and deliverable. All 4xx codes are temporary by definition. You only suppress on a 5xx permanent failure.

When to escalate: If a specific domain keeps returning 450s across multiple messages and multiple days, your IP or domain may be hitting a soft policy block on their end. That's different from standard greylisting. Check whether your sending domain has any reputation issues that might be triggering extended holds. You can run a quick blocklist check to rule that out.

If you're seeing 450s at volume across many recipients, that's a signal worth investigating beyond just retrying. Our Blocklist Checker is free and takes about 30 seconds. If something looks off, the SOS hotline is there for exactly this kind of situation.