Mail notification: unable to get local issuer certificate

Hi! Please help!
I entered my mail server settings

Mail server name:
Mail server port: 25
Mail server username (empty for none):
Mail server password: ********
Sender E-Mail Address:
Send mails only with SSL/TLS: 1
Check SSL/TLS certificate: 1
Server admin mail address:

When I try send test mail, I got
"Sending test mail failed. Error: Peer certificate cannot be authenticated with given CA certificates(ec=60), SSL certificate problem: unable to get local issuer certificate"

Working Thunderbird settings:

smtp server:
port: 25
auth method: secure

I understanding, that I need place my cert.pem, but I dont know where to put it.

Thank you for your help