Glossary
Protocol errors
An SMTP protocol error means the probe could not complete or interpret the command-and-reply exchange. The resulting check may remain inconclusive about the mailbox.
The exchange a probe must interpret
An SMTP connection begins with a server greeting, followed by commands and replies. Replies have three-digit codes and may span several lines, as defined in RFC 5321. The probe needs a readable, usable exchange to obtain recipient evidence. The SMTP handshake explanation describes the normal sequence.
Greeting, reply and reading-limit failures
SMTP_BANNER_INVALID means the opening reply did not supply the positive greeting the probe requires. A readable negative greeting can produce this code too. It does not establish that the port runs a non-mail service.
SMTP_PROTOCOL_ERROR can describe a malformed reply or another exchange the probe cannot complete or interpret. SMTP_REPLY_TOO_LARGE means a reply size, line count or accumulated session response exceeded a reading limit. Emvero stops reading at that limit. The limit concerns the exchange, not the mailbox’s storage capacity.
Other causes of an interrupted check
A correctly formatted 550 at RCPT TO is a recipient refusal and does not automatically indicate a protocol error. The ambiguous rejection explanation covers why its cause can remain unclear. An encryption-upgrade problem belongs to TLS and STARTTLS failures. A connection that never opened may reflect a transport failure or SMTP throttling.
Keeping the result in context
If no other evidence determines the decision, the completed result is unknown with allow_with_email_confirmation. An unusable exchange cannot establish that the mailbox is absent or that every mail route for the domain fails.
Keep reason_codes, checks and freshness with the address. If you manage the destination, repeated errors may justify reviewing its configuration. A later check may help after conditions change, without guaranteeing delivery. Unknown versus undeliverable explains when to use confirmation or separate review.
Related reason codes
The API uses these identifiers to explain a check. The notes identify values that the current verification policy or probe does not return.
SMTP_BANNER_ INVALID - Opening reply did not provide the required positive greeting
SMTP_PROTOCOL_ ERROR - SMTP exchange could not be completed or interpreted
SMTP_REPLY_ TOO_ LARGE - SMTP response exceeded a size, line-count or session reading limit