At the same time, check the postfix configuration, which can be viewed in the server status
"mail.135mailtest.com" is replaced with your domain name
Put:
smtpd_tls_chain_files=/etc/ssl/private/ssl-cert-snakeoil.key,/etc/ssl/certs/ssl-cert-snakeoil.pem
Change to:
smtpd_tls_chain_files = /www/server/panel/plugin/mail_sys/cert/135mailtest.com/privkey.pem,/www/server/panel/plugin/mail_sys/cert/135mailtest.com/fullchain.pem
Note to replace 135mailtest.com with your domain name.
Also check /www/server/panel/plugin/mail_sys/cert/135mailtest.com/ for privkey.pem and fullchain.pem files

You can use this command to check whether the correct SSL is configured
openssl s_client -connect mail.135mailtest.com:587 -starttls smtp
or
openssl s_client -connect mail.135mailtest.com:465

If the display is not OK, try to save the SSL certificate again, or please re-apply for SSL

aaP_randreevich

      aaPanel_Kern

      smtpd_tls_chain_files = /www/server/panel/plugin/mail_sys/cert/135mailtest.com/privkey.pem,/www/server/panel/plugin/mail_sys/cert/135mailtest.com/fullchain.pem

      The subdomain mail.135mailtest.com was specified
      And there was an error: Verify return code: 21 (unable to verify the first certificate)
      There is no error now, but Outlook says

      that the server does not support the specified connection encryption type.

          aaPanel_Kern
          smtpd_tls_chain_files = /www/server/panel/plugin/mail_sys/cert/example.com/privkey.pem,/www/server/panel/plugin/mail_sys/cert/mail.example.com/fullchain.pem

          indicate on the way example.com or mail.example.com ?

          After changing the path, should I recreate the certificate?

            I restored VPS from backup, updated the certificate again. I didn't change the line
            smtpd_tls_chain_files = /etc/ssl/private/ssl-cert-snakeoil.key,/etc/ssl/certs/ssl-cert-snakeoil.pem
            Google client doesn't give any errors.
            openssl s_client -connect mail.example.com:587 -starttls smtp
            checks pass without errors.
            It's not clear.