On 18/12/2020 08:22, Thomas Walter via mailop wrote:
Personally I'd use two A records for one name, but whatever. That's round robin, not "backup"? Systems will "randomly" connect to both connections and fail if one line is down - which was not the intention here.
No. It should try both. The order is random, but it should try both of them, so if one is down, it will use the other.
RFC 5321 - section 5.1 " When the lookup succeeds, the mapping can result in a list of alternative delivery addresses rather than a single address, because of multiple MX records, multihoming, or both. To provide reliable mail transmission, the SMTP client MUST be able to try (and retry) each of the relevant addresses in this list in order, until a delivery attempt succeeds."Note that the requirement to try each of the addresses is for multiple MX records *OR* multihoming (multiple A records)
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