Email guide
The difference between soft and hard bounces, what bounce rate is safe, and how to keep bounces from hurting your domain.
| Soft bounce | Hard bounce | |
|---|---|---|
| What it is | A temporary problem | A permanent failure |
| Error codes | Start with 4, such as 421 or 452 | Start with 5, such as 550 or 554 |
| Typical causes | Mailbox full, server busy, sending too fast, temporary rate limit | Address doesn’t exist, domain doesn’t exist, blocked for failing SPF, DKIM or DMARC |
| What to do | Retry later; remove after 3 to 5 soft bounces in a row | Remove the address straight away |
Some 5xx bounces aren’t about the address at all. 550 5.7.26 means your authentication failed, and 550 5.7.1 means you were judged as spam. Fix those at your end rather than deleting the contact.
Keep your bounce rate under 2% of emails sent. Above that, Gmail and Outlook start trusting your domain less, and some cold email tools can pause a campaign automatically when bounces climb. Above 5% is a sign the list itself is the problem.
Verify every list before you send. Old, bought and scraped lists are full of dead addresses.
Be careful with catch-all domains. They accept everything at first, then bounce later. Send to them in small batches.
Remove hard bounces automatically. Most sending tools can do this for you.
Slow down if soft bounces climb. It often means a provider is rate limiting you. See 421 4.7.28.
Check your authentication with the SPF checker, so 5xx blocks aren’t your fault.
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