I was unable to send emails.

This issue is caused by the support for STARTTLS for email sending, and occurs when a specific certificate is installed on the email server.

To resolve this issue, a checkbox called "Automatically renew STARTTLS communication" has been added to the mail server settings. This checkbox is checked by default, but if you are unable to send emails, you can uncheck it to enable email sending.

If you are using STARTTLS on a mail server that uses a certificate issued by your organization, you will be able to send emails even if the box is checked.