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more strict junk checks for some first-time senders: when TLS isn't used and when recipient address isn't in To/Cc header

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both cases are quite typical for spammers, and not for legitimate senders.
this doesn't apply to known senders. and it only requires that the content look
more like ham instead of spam. so legitimate mail can still get through with
these properties.
This commit is contained in:
Mechiel Lukkien
2023-11-27 10:34:01 +01:00
parent 8e37fadc13
commit 2ff87a0f9c
8 changed files with 69 additions and 19 deletions
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38
smtpserver/analyze.go
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@ -16,6 +16,7 @@ import (
"github.com/mjl-/mox/dns"
"github.com/mjl-/mox/dnsbl"
"github.com/mjl-/mox/iprev"
"github.com/mjl-/mox/message"
"github.com/mjl-/mox/mlog"
"github.com/mjl-/mox/mox-"
"github.com/mjl-/mox/publicsuffix"
@ -26,10 +27,13 @@ import (
)
type delivery struct {
tls bool
m *store.Message
dataFile *os.File
rcptAcc rcptAccount
acc *store.Account
msgTo []message.Address
msgCc []message.Address
msgFrom smtp.Address
dnsBLs []dns.Domain
dmarcUse bool
@ -369,11 +373,41 @@ func analyze(ctx context.Context, log *mlog.Log, resolver dns.Resolver, d delive
jitter := (jitterRand.Float64() - 0.5) / 10
threshold := jf.Threshold + jitter
// With an iprev fail, we set a higher bar for content.
rcptToMatch := func(l []message.Address) bool {
// todo: we use Go's net/mail to parse message header addresses. it does not allow empty quoted strings (contrary to spec), leaving To empty. so we don't verify To address for that unusual case for now. ../rfc/5322:961 ../rfc/5322:743
if d.rcptAcc.rcptTo.Localpart == "" {
return true
}
for _, a := range l {
dom, err := dns.ParseDomain(a.Host)
if err != nil {
continue
}
if dom == d.rcptAcc.rcptTo.IPDomain.Domain && smtp.Localpart(a.User) == d.rcptAcc.rcptTo.Localpart {
return true
}
}
return false
}
// todo: some of these checks should also apply for reputation-based analysis with a weak signal, e.g. verified dkim/spf signal from new domain.
// With an iprev fail, non-TLS connection or our address not in To/Cc header, we set a higher bar for content.
reason = reasonJunkContent
if suspiciousIPrevFail && threshold > 0.25 {
threshold = 0.25
log.Info("setting junk threshold due to iprev fail", mlog.Field("threshold", 0.25))
log.Info("setting junk threshold due to iprev fail", mlog.Field("threshold", threshold))
reason = reasonJunkContentStrict
} else if !d.tls && threshold > 0.25 {
threshold = 0.25
log.Info("setting junk threshold due to plaintext smtp", mlog.Field("threshold", threshold))
reason = reasonJunkContentStrict
} else if (rs == nil || !rs.IsForward) && threshold > 0.25 && !rcptToMatch(d.msgTo) && !rcptToMatch(d.msgCc) {
// A common theme in junk messages is your recipient address not being in the To/Cc
// headers. We may be in Bcc, but that's unusual for first-time senders. Some
// providers (e.g. gmail) does not DKIM-sign Bcc headers, so junk messages can be
// sent with matching Bcc headers. We don't get here for known senders.
threshold = 0.25
log.Info("setting junk threshold due to smtp rcpt to and message to/cc address mismatch", mlog.Field("threshold", threshold))
reason = reasonJunkContentStrict
}
accept = contentProb <= threshold