Reports the SMTP endpoint, the fixed sender address, who this server is allowed to write to, the current limits and whether sending is switched on at all. Call this first: it answers "can I send, and to whom" without touching the network.

Says which of the given addresses this server is allowed to write to, and why the others are refused. Nothing is sent and no connection is made. Use it before composing a message rather than discovering the refusal afterwards.

Builds exactly the message a send would build and returns its headers and bodies, without connecting to anything. Every check a send performs runs here too — the allowlist, the recipient limit, the attachment policy and the size limit — so this is the way to find out whether a message is acceptable before asking a human to approve it. The sending tools register only when SMTP_ALLOW_SEND is true and may therefore be absent even where this preview succeeds; get_server_info reports whether sending is on. Attachment payloads are summarised by name, size and digest rather than printed.

Opens a connection to the SMTP server, negotiates TLS and authenticates, then closes it again. No message is sent. Use it to tell a configuration problem apart from a delivery problem. Tries the server at most once every ten seconds; a call inside that window repeats the previous outcome and says so.