DE

↑ ↓ select, Enter open, Esc close

Test an SMTP connection with STARTTLS and certificate

harmlessMaildebian, ubuntu
Adjust the values, the command updates live
user@server
openssl s_client -connect mail.example.com:587 -starttls smtp -servername mail.example.com

Connects to the mail server’s submission port, switches to encryption via STARTTLS and prints the certificate chain, validity and negotiated cipher. Useful when mail clients show certificate warnings or sending fails at the TLS handshake. Exit with QUIT or Ctrl+C.

Note: For port 25 the command stays the same, for IMAP the variant is -connect mail.example.com:143 -starttls imap.

Also searched as

  • check mail server certificate
  • outlook certificate error when sending
  • starttls not working

Related one-liners

All in Mail

Check the SSL certificate expiration date of a website

$openssl s_client -connect example.com:443 -servername example.com </dev/null 2>/dev/null | openssl x509 -noout -dates
harmlessSSL and certificates

Check whether a certificate expires within the next days

#openssl x509 -in /etc/letsencrypt/live/example.com/cert.pem -noout -checkend $(( 30 * 86400 )) && echo "noch gültig" || echo "läuft bald ab"
harmlessSSL and certificates

Check whether a private key matches a certificate

#openssl x509 -noout -modulus -in /etc/ssl/certs/example.com.crt | openssl md5 && openssl rsa -noout -modulus -in /etc/ssl/private/example.com.key | openssl md5
harmlessSSL and certificates

Check whether a server supports TLS 1.3

$openssl s_client -connect example.com:443 -servername example.com -tls1_3 </dev/null 2>/dev/null | grep -E "^New,|Protocol *:"
harmlessSSL and certificates

Check whether a website's certificate is trusted

$openssl s_client -connect example.com:443 -servername example.com -verify_hostname example.com </dev/null 2>/dev/null | grep "Verify return code"
harmlessSSL and certificates

Create a self-signed certificate for testing

$openssl req -x509 -newkey rsa:2048 -nodes -days 365 -keyout test.key -out test.crt -subj "/CN=test.example.com" -addext "subjectAltName=DNS:test.example.com"
cautionSSL and certificates

Read first, then run.

The commands on myline.de act directly on servers, files and databases. A wrong path or placeholder can delete data irreversibly or make a server unreachable.

  • All commands are provided without warranty and are not tested on every system.
  • Understand what a command does before running it, and check every placeholder.
  • Make a backup first and, if possible, try it on a test system.
  • You run commands at your own risk. Liability for damages is excluded to the extent permitted by law.