STARTTLS offered

STARTTLS upgrades a plaintext SMTP session to TLS in place. This check reads the EHLO response from each reachable MX host and reports whether STARTTLS is among the capabilities it advertises. A server that does not offer it receives mail in the clear, readable by anything on the path.

It is a warning rather than a failure because inter-server TLS is opportunistic by design: a sender that cannot negotiate TLS almost always delivers anyway rather than holding the message. That is a deliberate choice — mail arriving unencrypted is judged better than mail not arriving — and it is exactly why the absence is worth fixing rather than tolerating.

It is also why opportunistic TLS alone is not confidentiality. An attacker on the path can strip the STARTTLS capability out of the greeting, and the sender, seeing no offer, delivers in plaintext with nothing looking wrong at either end. Closing that requires the sender to know in advance that TLS is required, which is what MTA-STS and DANE are for.

Implicit TLS on port 465 does not substitute for this. That port is for message submission by users; server-to-server delivery arrives on port 25 and negotiates with STARTTLS or not at all.

Enable STARTTLS on port 25 and give the server a certificate covering the MX hostname. A free ACME certificate is fine — for opportunistic TLS the name barely matters today, and it matters completely the moment you publish MTA-STS or DANE.

Do not require TLS for inbound mail on port 25. Refusing plaintext senders loses mail from a long tail of old systems, and the right way to require TLS is to tell senders to require it — publish an MTA-STS policy, or a TLSA record if you run DNSSEC — rather than to refuse them at the door.

Restrict the protocol versions and ciphers rather than the availability: TLS 1.2 as a floor, 1.3 where you can. Then publish a TLS-RPT record, because senders reporting failed negotiation is the only way you will hear about one.

Keep user submission on port 587 with STARTTLS and authentication required, separate from the port 25 listener. Mixing the two is how servers end up advertising AUTH in cleartext to the whole internet.