SMTP Codes

smtp.codes is a collection of SMTP response codes returned by email servers and a guide to help you understand what the SMTP codes that various email servers respond with mean.

This is super useful when troubleshooting email delivery and a tool every email marketer, deliverability person or email engineer should have in their tool kit. We welcome all contributions!

Codes

• 421
• 422
• 450
• 451
• 452
• 501
• 521
• 540
• 550
• 551
• 552
• 553
• 554

Providers

• Barracuda
• Cisco
• Comcast
• Email Security.cloud
• FireEye
• Free.fr
• GoDaddy
• Google
• HEY
• Hostpoint
• iCloud
• ImprovMX
• Infomaniak
• La Poste
• Mail.ru
• Microsoft 365
• Mimecast
• MXroute
• Orange
• Outlook.com
• Proofpoint
• Proton Mail
• QQ
• Rackspace
• Rambler
• Titan Email
• Trend Micro
• t-online
• United Internet (1&1, IONOS, GMX, web.de, mail.com)
• Yahoo
• Yandex
• Zoho