From SMTP to TLS
STARTTLS adds TLS to an existing SMTP connection. After EHLO, a server may advertise STARTTLS; TLS negotiation starts after acceptance. SMTP information is obtained again afterwards. On port 25 this concerns transport between mail servers, not signing in from your email application.
What our check does
The optional check runs from Düsseldorf and inspects at most two MX targets. One public address is used per target. Certificate, mail server name and trust chain are verified during the TLS connection. The tool shows the negotiated protocol and certificate expiry. It does not authenticate or send an email.
Interpreting results
A connection failure can also result from outbound port-25 restrictions on the checking server. Missing STARTTLS describes this connection. Other addresses, servers or measurement locations may differ. Even a positive result guarantees no delivery and assesses neither DANE nor MTA-STS.
Further investigation
Sending from an email application usually involves other ports and authentication settings. This version specifically checks receiving mail targets on port 25. Compare findings with server logs and provider instructions before changing settings.