DNS & the next hop
02A recipient domain leads to DNS mail exchangers, then usable host addresses for a delivery attempt.
Related RFCs and changes
“Updates” changes part of an earlier RFC. “Obsoletes” replaces it. This list shows selected related documents.
DIAGRAM§ 5.1 ↗
RFC 5321 · § 5.1EXAMPLE
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04
Handle an unreachable candidate.
Appropriate alternatives are tried before the sender treats the delivery attempt as exhausted.
Protocol participantThreat or exceptionProtocol controlSimplified drawings · precise section references
Read the updated RFC section ↗
Resolve mail routing before opening the next connection.
- MX preference values rank mail exchangers; lower values are preferred.
- With no MX records, the domain can act as an implicit MX under §5.1 rules.
- Try appropriate alternative addresses and hosts; a temporary DNS failure is different from an authoritative nonexistence result.
MX targets need usable address records. The relay must also avoid routing loops. Later null-MX conventions are defined separately, rather than being a feature invented by RFC 5321.
Read the RFC section ↗Diagrams simplify the RFC. Read the linked sections for complete requirements.
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