I currently run a Plesk VPS, with 2 Domains hosted within.

Some time ago, I had installed the 'Let's Encrypt' SSL Certificate onto my Plesk VPS. Both domains, successfully displayed the 'https' prefix within the domain and I was able to use the emails with no problems, within Outlook 2016.

Recently, I have started to receive the following 'Internet Security Warning' whenever I try to add new emails into Outlook 2016.

I have spoken with Plesk, my hosting company. Microsoft and Let's Encrypt, with all parties stating differing errors, explaining I need to speak with the other party.

Is anyone able to point me in a possible direction, as to resolve this issue?

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  • 1
    Is it a subject alternative name (SAN) certificate? Based on the error message it sounds like outlook 2016 may not support alternate names. It sounds like it is only looking at the main (principal) name. Jan 17, 2018 at 2:06
  • Are you hosting mail on your VPS, or are you using Plesk's shared mail services? If you are using shared services you probably need to change your mail server settings to use something like mail.plesk.tld rather than mail.mydomain.tld. See: msoutlook.info/question/613 Jan 17, 2018 at 2:11
  • I am hosting the emails within both domains, on the same VPS. Not sure if this would be an issue but I have entered 'mail.domain.com' into the DNS Records. Would I need to create a sub domain for this to work? I have no problems sending and receiving emails, just want to ensure I get the certification set up correctly.
    – Craig
    Jan 17, 2018 at 2:18

1 Answer 1


I have managed to resolve the issue.

Step One:

When you first see the 'Internet Security Warning', select 'View Certificate'.

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Step Two:

What you then need to do, is identify who the Certificate has been issued to. Refer to the below image, for where to find this information. Once you have identified the location, copy the entry exactly as is.

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Step Three:

Head into your Outlook and then head into File > Account Settings > Account Settings. You should then be on a page, which looks something like:

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Simply select one of your emails by double clicking on the relevant account.

Step Four:

Referring to Step Two, simply paste in the 'Issued to' information into the Incoming mail server and Outgoing mail server(SMTP) text boxes, as highlighted in the below image.

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Step Five:

All that is left to do now, is to click 'Next' and continue as you would normally. Then repeat the process for the other emails.

Where I was going wrong, was that I had mail.domain.com entered into the Incoming and Outgoing server text boxes. This was ok for a while, since I hadn't installed an SSL Certificate onto my server but now that I have an SSL Certificate, I needed to change my entries, to reflect the name of my server, which the SSL Certificate was installed on.

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