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refactor(transport): Explicitly handle RFC7505 null MX

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Signed-off-by: Jagoda Ślązak <jslazak@jslazak.com>
This commit is contained in:
Jagoda Ślązak
2026-04-28 08:24:27 +02:00
parent 21aaea0d92
commit 1b1585eb7d
1 changed files with 17 additions and 8 deletions
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filtermail/src/transport.rs
+17 -8
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@@ -62,14 +62,23 @@ impl TransportHandler {
match dns_resolver.mx_lookup(query).await {
Ok(mx_records) => {
let mut hosts: Vec<(u16, String)> = mx_records
.iter()
.map(|mx| {
let host =
mx.exchange().to_string().trim_end_matches('.').to_string();
(mx.preference(), host)
})
.collect();
let mut hosts: Vec<(u16, String)> = Vec::new();
for mx in mx_records {
// Null MX / RFC7505
if mx.exchange().is_root() {
// From RFC7505 section 3:
// > A domain that advertises a null MX MUST NOT
// > advertise any other MX RR.
// We assume this is the only record and exit early.
return Err(
"556 5.1.10 Permanent failure: Recipient address has null MX"
.to_string(),
);
}
let host = mx.exchange().to_string().trim_end_matches('.').to_string();
hosts.push((mx.preference(), host))
}
hosts.sort();
hosts
}