Recipient verification and how MxGuard stops backscatter

Backscatter is the flood of misdirected bounce messages a mail system generates when it accepts mail for addresses that don't exist and then tries to bounce it later. It clogs queues and can get your domain blocklisted.

MxGuard avoids it entirely. At SMTP time, during the Recipient stage of the pipeline, MxGuard sends a verification probe to your real mail server to confirm that the recipient address genuinely exists before accepting the message. If the address is invalid, the sending server is rejected at the SMTP conversation — so there is no orphaned message to bounce later.

To keep this fast and to avoid hammering your backend, verification results are cached (backend probe caching). The net effect: no backscatter, no accept-then-bounce, and no unnecessary load on your mail server for repeated lookups of the same address.