Please install the SSL certificate on MailEnable webmail first and Configure your SSL certificate to your mail domain in IIS.
> Please open the Mailenable
> Please right-click on Server> localhost and click the properties button.
> Please select the SSL tab and select the installed SSL certificate.
> Please Expand Service and Connectors, Right Click on SMTP, and Click on Properties.
> Please click on Inbound Tab. Tick on Allow Clients to establish secure sessions via STARTTLS.
> Please click on the apply button Open Port Settings. please set port 465 Click OK to save the changes.
> Please restart SMTP and Right Click on IMAP > Properties. Please check on also listening on alternative port: 993 and also check the required SSL
> Please restart IMAP service and right-click on pop> properties, Please check on also listening on alternative port: 993 and also check the required SSL
> Please restart POP Service. Please check the SMTP Outbound TLS setting and IMAP setting.
> Please send a test message to your Gmail and check if TLS encryption is enabled.