How do I create an SSL Certificate form the 2 different Mail Servers, within the same VPS?

I currently run a Plesk VPS, which host 2 domains. I have installed a Let’s Encrypt Certificate onto the VPS as well as within both of the domains.

Upon installation, the ‘Secure’ notification and valid Let’s Encrypt Certification appear in the browser for both the VPS and the 2 domains; which is exactly what I want.

Unfortunately, I have now come into a problem with the emails. I can send and receive emails perfectly fine. This is the case for when I directly log in via Webmail and when I use an email client such as Outlook 2016.

That said, I seem to encounter a problem with the email certificate. When I go to add an email account to Outlook 2016, I encounter the following Internet Security Warning notification within Outlook 2016:


The certificate displays ‘’ (Not the real name), which is the name of the Plesk Domain and is inserted into the Let’s Encrypt Certificate.

I accessed to above by going to: Plesk Control Panel > Tools & Settings > SSL/TLS Certificates > Let’s Encrypt.

Could the issue be that I am trying to apply the Server certificate to the domain email rather than having a separate Mail Domain Server certificate? I have already created 2 domains, where I have successfully uploaded the certificate.

If this is the case, would I need to create a certificate for ‘mail.domain.tld’? If this is the case, what would be the steps for this? I have had conflicting steps across various forums. Would I need to create a Sub Domain for the emails? Or change my Hostname etc?

Through looking at many articles, over the past few days, I have completely confused myself over the application of the SSL Certificate by Let’s Encrypt.

Any direction, on this matter, would be greatly appreciated.

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