openssl s_client -connect mail.tools.wmflabs.org:25 -starttls smtp outputs:
CONNECTED(00000003) depth=0 CN = mail.tools.wmflabs.org verify error:num=20:unable to get local issuer certificate verify return:1 depth=0 CN = mail.tools.wmflabs.org verify error:num=21:unable to verify the first certificate verify return:1 --- Certificate chain 0 s:/CN=mail.tools.wmflabs.org i:/C=US/O=Let's Encrypt/CN=Let's Encrypt Authority X3 --- Server certificate -----BEGIN CERTIFICATE----- MIIEmjCCA4KgAwIBAgISA3CQV+TEuTEUJdFrfPfEXdMmMA0GCSqGSIb3DQEBCwUA MEoxCzAJBgNVBAYTAlVTMRYwFAYDVQQKEw1MZXQncyBFbmNyeXB0MSMwIQYDVQQD ExpMZXQncyBFbmNyeXB0IEF1dGhvcml0eSBYMzAeFw0yMDA2MjMxNTA2MzNaFw0y MDA5MjExNTA2MzNaMCExHzAdBgNVBAMTFm1haWwudG9vbHMud21mbGFicy5vcmcw WTATBgcqhkjOPQIBBggqhkjOPQMBBwNCAATZLb0lH6NvZWRuhZS3h4Fh8HuxgDhp 3FaiAuj/o1/jv9fCk+yjIW0gNus0L8Y1Hrvufz8nJb/Tt8LaP3eKv0BHo4ICbDCC AmgwDgYDVR0PAQH/BAQDAgeAMB0GA1UdJQQWMBQGCCsGAQUFBwMBBggrBgEFBQcD AjAMBgNVHRMBAf8EAjAAMB0GA1UdDgQWBBTlsZoULnb33wrco/85KhEF0WTGlDAf BgNVHSMEGDAWgBSoSmpjBH3duubRObemRWXv86jsoTBvBggrBgEFBQcBAQRjMGEw LgYIKwYBBQUHMAGGImh0dHA6Ly9vY3NwLmludC14My5sZXRzZW5jcnlwdC5vcmcw LwYIKwYBBQUHMAKGI2h0dHA6Ly9jZXJ0LmludC14My5sZXRzZW5jcnlwdC5vcmcv MCEGA1UdEQQaMBiCFm1haWwudG9vbHMud21mbGFicy5vcmcwTAYDVR0gBEUwQzAI BgZngQwBAgEwNwYLKwYBBAGC3xMBAQEwKDAmBggrBgEFBQcCARYaaHR0cDovL2Nw cy5sZXRzZW5jcnlwdC5vcmcwggEFBgorBgEEAdZ5AgQCBIH2BIHzAPEAdgDwlaRZ 8gDRgkAQLS+TiI6tS/4dR+OZ4dA0prCoqo6ycwAAAXLh7lrIAAAEAwBHMEUCIFDh jsTfCm1ILSgi3ZKnuFms/lmOi+sSta5cfUBQSaYFAiEAlcjjAgWXCVX8PSe9Mx9s EZ6n2mNnEjrUdR0Zs5zRiSAAdwAHt1wb5X1o//Gwxh0jFce65ld8V5S3au68YToa adOiHAAAAXLh7lr3AAAEAwBIMEYCIQDdZMb0rDjUR68a2wELpn6t78mSPXDZ5NGD Nv0gFvZtmAIhANo35YavWGskBp+vwkHcaTDbIpdJATtsliiiGwpkdNi6MA0GCSqG SIb3DQEBCwUAA4IBAQATO5KBp110TjvVtoWoumkPhPZjNb60ygZ5nChZ1e2FY1+L UAEw7k2r/hKWYbD4cfXZ6dEJtzlSVDWvXpej0pD+R3GWbrJFkQAESA2wQFACnfh1 FmAvI3+pHEPoKv+RCRrxoUGFcL+Nc3MloBABe6mbXkrtc1Qm40rxZ5ZW8kMZ5lrR I3E7cCx6NTOG9+NLLHHLM+5CR81kx5lnrQC6SpTmDOSFXhNialG1piE1smqgjBVI 9eePHLLVh5hzYqK/Ih7z77qA3YeJFNI1o/odnq5I+x188kYWfhNVe1Dzzvv1RG1Y exDMvI1TjSWCODL6idZujOO/Tu1lcK/wTEIMczkC -----END CERTIFICATE----- subject=/CN=mail.tools.wmflabs.org issuer=/C=US/O=Let's Encrypt/CN=Let's Encrypt Authority X3 --- No client certificate CA names sent Peer signing digest: SHA512 Server Temp Key: ECDH, P-256, 256 bits --- SSL handshake has read 1721 bytes and written 335 bytes Verification error: unable to verify the first certificate --- New, TLSv1.2, Cipher is ECDHE-ECDSA-AES256-GCM-SHA384 Server public key is 256 bit Secure Renegotiation IS supported Compression: NONE Expansion: NONE No ALPN negotiated SSL-Session: Protocol : TLSv1.2 Cipher : ECDHE-ECDSA-AES256-GCM-SHA384 Session-ID: 6E1DB07FFC3974A597BDA6C942EE025BEF2E7D6759C2E020C23C0049FA75A220 Session-ID-ctx: Master-Key: A072F47C634E9B4631FF592B90E72F691DE5D32A6E2309AF9B5044FBDE22A7B45957E30CEF208B3EAD39ED0B6A456A20 PSK identity: None PSK identity hint: None SRP username: None Start Time: 1593547835 Timeout : 7200 (sec) Verify return code: 21 (unable to verify the first certificate) Extended master secret: yes --- 250 HELP
The certificate chain is ending early, at the intermediate certificate, and not landing correctly at the Let's Encrypt root like it should. This means any client trying to send mail with STARTTLS will fail.