Unable to send mail via SSL: warning: TLS library problem: 31990

Symptoms

• Unable to connect mail client on SMTPS port 465/993 SSL/TLS connection. Mail client displays the error:

  Your server has unexpectedly terminated the connection. Possible causes for this include server problems, network problems, or a long period of inactivity. Account: 'my email', Server: '3.4.5.6', Protocol: SMTP, Port: 465, Secure(SSL): Yes, Error Number: 0x800CCC0F

• Following error message is present in /usr/local/psa/var/log/maillog:

  postfix/smtpd[31990]: warning: TLS library problem: 31990:error:14094418:SSL routines:SSL3_READ_BYTES:tlsv1 alert unknown ca:s3_pkt.c:1197:SSL alert number 48:
  postfix/smtpd[31990]: lost connection after CONNECT from unknown[1.2.3.4]
  postfix/smtpd[31990]: disconnect from unknown[1.2.3.4]

• Mail client displays the following error:

  Your server has unexpectedly terminated the connection. Possible causes for this include server problems, network problems, or a long period of inactivity. Account: 'my email', Server: '3.4.5.6', Protocol: SMTP, Port: 465, Secure(SSL): Yes, Error Number: 0x800CCC0F

Cause

SSL CA certificates bundle is missing or corrupted.

Resolution

1. Install a valid certificate using the article Plesk following the instructions here: How to secure mail server with Let's Encrypt certificate

2. Connect to the server via SSH.

3. Run the following command:

   # plesk repair mail

4. In case this issue still persists, reinstall ca-certificates package.

   For example, for CentOS 6:

   # rpm -Uhv ca-certificates-2010.63-3.el6_1.5.noarch