Ambiguous rejection

Why a 550 reply needs context

An SMTP 550 reply says that the requested action was not taken. RFC 5321 lists possible causes including a missing mailbox, denied access and a policy refusal. The number alone cannot tell a verifier which cause applies.

The command matters too. At RCPT TO, the server has heard the recipient address. A refusal before that command concerns an earlier step, such as the envelope sender. The policy rejection explanation covers those restrictions.

An enhanced status code can help distinguish an addressing problem, mailbox condition or policy rule. RFC 3463 defines mailbox-status conditions separately from other categories. The extra detail helps classify the probe response, but cannot establish ownership or an actual delivery outcome. The hard and soft bounce explanation covers sending failures.

How Emvero reports recipient refusals

SMTP_RECIPIENT_REJECTED_AMBIGUOUS returns unknown with allow_with_email_confirmation. The current policy also converts an observation classified as SMTP_MAILBOX_NOT_FOUND to this result and reason. A provider profile cannot enable a separate rejection rule for that observation. Unknown versus undeliverable explains how this differs from deterministic syntax or domain failures.

SMTP_MAILBOX_STATUS_ERROR identifies a mailbox-status problem, and SMTP_MAILBOX_DISABLED identifies a disabled mailbox. These return risky with the confirmation recommendation. The cause is more specific, but the observation cannot tell you when the address will receive mail again.

Choosing an application or sending action

If access matters at signup, have your application send confirmation; Emvero sends no verification message. The signup validation guide explains how to apply the recommendation. When reviewing a list, keep the address and refusal reason together until your sending policy resolves the decision. The verification result guide shows the complete response.