Page 217 of the DSE Security Knowledge Hub, with source-backed guidance, checklists, explainers, and playbooks.
A delivery-status notification is a multipart/report with report-type delivery-status, a human-readable part, and a message/delivery-status part; returned original content is optional.
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A sender discovers MTA-STS through the policy domain’s _mta-sts TXT record, whose id value distinguishes policy revisions. In enforce mode, the sender requires STARTTLS and valid MX identity for each candidate, while a still-valid cached policy remains applicable when live retrieval fails.
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Content-Disposition says whether a MIME body part is intended for automatic inline presentation or for user-initiated attachment handling; it does not define the media format.
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A Mailbox is a named collection of Email objects, and moving an Email between Mailboxes does not change the Email object’s identifier. An Email object represents one RFC 5322 message independently of the Mailbox and Thread objects that organize or group it.
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After a 220 response to STARTTLS, the client begins the TLS handshake before sending any further SMTP command. A successful TLS handshake resets SMTP to its initial state; both peers discard pre-TLS protocol knowledge and the client sends a new EHLO.
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An SMTP server advertising SMTPUTF8 accepts UTF-8 wherever the SMTP mailbox grammar permits a mailbox value. A client may send internationalized mailbox names and UTF-8 headers only after the server advertises SMTPUTF8 in its EHLO response.
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The RCPT NOTIFY parameter requests DSNs for SUCCESS, FAILURE, or DELAY; NEVER must appear alone and requests no DSN for that recipient. The RCPT ORCPT parameter preserves the original recipient address that corresponds to the actual delivery recipient.
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An enhanced mail status code contains three dot-separated fields: delivery class, problem subject, and specific detail. Class 2 means success, class 4 means a persistent transient failure that may succeed later, and class 5 means a permanent failure requiring a change.
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A multipart body is divided by boundary lines into parts, each containing its own content headers, a blank line, and a body. The multipart Content-Type requires a boundary parameter, and the chosen delimiter must not occur as a delimiter line inside any encapsulated part.
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The UTF-8 address type has native, unitext, and ASCII-safe xtext forms; native UTF-8 cannot be used through a server that lacks SMTPUTF8. A message/global-delivery-status part uses UTF-8 and should preserve non-ASCII recipient addresses in the native UTF-8 address form.
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