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give delivering to mx targets with underscores in name a chance of succeeding

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the underscores aren't valid, but have been seen in the wild, so we have a
workaround for them. there are limitations, it won't work with idna domains.
and if the domain has other policies, like mta-sts, the mx host won't pass
either.

after report from richard g about delivery issue, thanks!
This commit is contained in:
Mechiel Lukkien
2023-10-25 13:01:11 +02:00
parent 682f8a0904
commit d1e93020d8
4 changed files with 68 additions and 13 deletions
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3
smtpclient/gather.go
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@ -148,7 +148,8 @@ func GatherDestinations(ctx context.Context, log *mlog.Log, resolver dns.Resolve
// The Go resolver already sorts by preference, randomizing records of same
// preference. ../rfc/5321:3885
for _, mx := range mxl {
host, err := dns.ParseDomain(strings.TrimSuffix(mx.Host, "."))
// Parsing lax (unless pedantic mode) for MX targets with underscores as seen in the wild.
host, err := dns.ParseDomainLax(strings.TrimSuffix(mx.Host, "."))
if err != nil {
// note: should not happen because Go resolver already filters these out.
err = fmt.Errorf("%w: invalid host name in mx record %q: %v", errDNS, mx.Host, err)