On Thu, Oct 30, 2025 at 04:00:45PM +0000, John Levine via mailop wrote:
> It appears that Edmund Lodewijks via mailop <[email protected]> said:
>
> >Does this setup work, a self-signed certificate in combination with
> >DANE?
> >
> >Whenever I tried this, connections from Gmail and Protonmail (and
> >potentially others) got dropped right after tls:
There is a non-trivial minority of SMTP servers that have self-signed
certificates. Barring some reason to expect otherwise (MTA-STS policy?),
SMTP clients broadly tolerate unvalidatable server certificates,
including self-signed as one of many ways that can happen. That's just
basic unauthenticated opportunistic TLS.
For example, the backup MX of the Armenian NIC, "isoc.amnic.net" has
a self-signed cert and matching TLSA records:
_25._tcp.isoc.amnic.net. IN TLSA 3 1 1
e85440f7099608bfaf645ce0b4806f556601a96f4a720c91b6e76ea4cdd0d4e6
isoc.amnic.net[195.43.74.26]: pass: TLSA match: depth = 0
TLS = TLS1.3 with TLS_AES_256_GCM_SHA384,X25519,PubKeyALG_RSA
name = isoc-1.amnic.net
depth = 0
Issuer CommonName = isoc-1.amnic.net
Issuer Organization = Internet Society Public Organization
notBefore = 2024-07-04T18:10:52Z
notAfter = 2027-07-04T18:10:52Z
Subject CommonName = isoc-1.amnic.net
Subject Organization = Internet Society Public Organization
pkey sha256 [matched] <- 3 1 1
e85440f7099608bfaf645ce0b4806f556601a96f4a720c91b6e76ea4cdd0d4e6
Likewise both MX hosts of a Czech health insurance provider:
cpzp.cz. IN MX 10 posta.cpzp.cz.
cpzp.cz. IN MX 30 postc.cpzp.cz.
_25._tcp.posta.cpzp.cz. IN TLSA 3 1 1
582364ed6dd790dbb5c121050ab0620c9be20a8d7e59ed55a2e917b744028165
posta.cpzp.cz[193.85.230.100]: pass: TLSA match: depth = 0, name =
posta.cpzp.cz
TLS = TLS1.2 with TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384,X25519
name = posta
name = posta.cpzp.cz
depth = 0
Issuer CommonName = posta
notBefore = 2021-09-21T16:05:32Z
notAfter = 2026-09-21T16:05:32Z
Subject CommonName = posta
pkey sha256 [matched] <- 3 1 1
582364ed6dd790dbb5c121050ab0620c9be20a8d7e59ed55a2e917b744028165
_25._tcp.postc.cpzp.cz. IN TLSA 3 1 1
cedc8c091d393f523c0ab409cd06ff6b0f0b15ae1acf83990585328b2dc44200
postc.cpzp.cz[193.85.230.101]: pass: TLSA match: depth = 0, name =
postc.cpzp.cz
TLS = TLS1.2 with TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384,X25519
name = postc
name = postc.cpzp.cz
depth = 0
Issuer CommonName = postc
notBefore = 2021-07-01T06:46:53Z
notAfter = 2026-07-01T06:46:53Z
Subject CommonName = postc
pkey sha256 [matched] <- 3 1 1
cedc8c091d393f523c0ab409cd06ff6b0f0b15ae1acf83990585328b2dc44200
So you should be able to field a DANE-validatable self-signed cert and
not have trouble receiving mail from either non-DANE SMTP clients or
DANE-enabled SMTP clients (provided the TLSA records match that cert or
its public key).
> It works for mail systems that use DANE.
No, it is expected to just work regardless. Some clients might balk if
the certificate is manifestly expired (a mistake, but best to not tempt
them). Some might also object to poor choices of key usage or extended
key usage extensions.
> I know Gmail doesn't (that is why we have MTA-STS). Protonmail's DNS
> isn't DNSSSEC signed, so I doubt they do either.
The protomail.ch domain is not only DNSSEC-signed, but its MX hosts even
have TLSA records, and support over 82K DNSSEC-signed customer domains.
So they definitely to *inbound* DANE. I was under the impression they
also do outbound DANE, but I not in possession of hard facts that
demonstrace that to be the case.
https://stats.dnssec-tools.org/explore/?protonmail.ch
Though in this case not self-signed:
_25._tcp.mail.protonmail.ch. IN TLSA 3 1 1
6111a5698d23c89e09c36ff833c1487edc1b0c841f87c49dae8f7a09e11e979e
_25._tcp.mail.protonmail.ch. IN TLSA 3 1 1
76bb66711da416433ca890a5b2e5a0533c6006478f7d10a4469a947acc8399e1
mail.protonmail.ch[176.119.200.128]: pass: TLSA match: depth = 0, name =
*.protonmail.ch
TLS = TLS1.3 with TLS_AES_256_GCM_SHA384,X25519,PubKeyALG_RSA
name = *.pm.me
name = *.protonmail.ch
name = *.protonmail.com
name = *.protonvpn.ch
name = *.protonvpn.com
name = protonmail.com
depth = 0
Issuer CommonName = R13
Issuer Organization = Let's Encrypt
notBefore = 2025-10-07T13:23:55Z
notAfter = 2026-01-05T13:23:54Z
Subject CommonName = protonmail.com
pkey sha256 [matched] <- 3 1 1
76bb66711da416433ca890a5b2e5a0533c6006478f7d10a4469a947acc8399e1
depth = 1
Issuer CommonName = ISRG Root X1
Issuer Organization = Internet Security Research Group
notBefore = 2024-03-13T00:00:00Z
notAfter = 2027-03-12T23:59:59Z
Subject CommonName = R13
Subject Organization = Let's Encrypt
pkey sha256 [nomatch] <- 2 1 1
025490860b498ab73c6a12f27a49ad5fe230fafe3ac8f6112c9b7d0aad46941d
All five of the domains in the certifiate are DNSSEC-signed and have
working inbound DANE: pm.me, protonmail.ch, protonmail.com,
protonvpn.ch, protonvpn.com.
--
Viktor. 🇺🇦 Слава Україні!
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