SMTP Error Codes
SMTP Error Codes by Spam Filter
Spam filters and security gateways sit in front of the inbox and can reject mail before the mailbox provider ever sees it. These pages collect the exact SMTP rejection text from the major filtering services.
All spam filters
- Barracuda6 codesBarracuda is a mail security gateway for businesses. Mail routes through Barracuda before the recipient's server, and while recipients manage their own allowlists, a reputation issue affecting all Barracuda customers has to go through Barracuda support.
- Email Security.cloud4 codesEmail Security.cloud is a Broadcom (formerly Symantec) filtering service that works alongside the Symantec Messaging Gateway, with detailed troubleshooting guides and a submission path for false positives.
- Exchange Online Protection3 codesExchange Online Protection is the filtering layer in front of every Microsoft 365 mailbox, providing spam filtering, blocklists and connection filtering. Specific errors usually map to a documented fix in Microsoft's guidance.
- FireEye1 codeFireEye, now part of Trellix, is a mail security gateway that filters mail through its own servers before delivery, with recipient-managed allowlists and policy controls.
- Mimecast3 codesMimecast is a corporate mail security service. Inbound mail routes through Mimecast servers before the recipient's, and the recipient controls policy levels and allowlist overrides.
- Proofpoint3 codesProofpoint is a corporate mail security gateway. Inbound mail passes through Proofpoint-owned servers before final delivery, recipients control their own allowlists and policy levels, and IP-level blocks are appealed through Proofpoint directly.
- Trend Micro2 codesTrend Micro provides mail security and reputation filtering for businesses, routing mail through its own servers before delivery. Recipients manage allowlists and policy settings.
- ZeroSpam2 codesZeroSpam is a Canadian mail security gateway. Mail passes through its servers before delivery, recipients manage allowlists, and senders can reach its postmaster and abuse addresses for a closer look.
