Windows Server 2012 R2
IIS 8.5
DNN 7.4 (DotNetNuke)
letsencrypt-win-simple

Heya everyone

I have multiple sites running from our self managed Azure VPS using the DNN website CMS.

We have recently installed and started using the letsencrypt-win-simple installer for providing Lets Encrypt SSL Certs to some of our sites and this has gone very smoothly and is by and large working very well, thanks to everyone involved!

The issue is that some sites don't currently want/need to have an SSL cert for a variety of reasons. no problem you say, just leave them and carry on, well this is what I thought too however it appears that this is causing an issue with autodiscover for office 365 on sites that don't have SSL certs. This comes to light when autodiscover is looking initially for an answer at https://domain.com:443/Autodiscover/Autodiscover.xml when the site has no cert. instead it is showing the cert of another domain ( I guess by using a revers IP look up?

Site1.com (has Cert, this appears to be what ever domain I have most recently installed a cert for)
Site2.com (no Cert)

https://Site2.com:443/Autodiscover/Autodiscover.xml returns an error of This server could not prove that it is Site2.com; its security certificate is from Site1.com. This may be caused by a misconfiguration or an attacker intercepting your connection.

Short term solution here is to install a cert for Site2.com but that doesn't really fix the issue and I'm sure will continue to rear it head as there are many domain sites hosted on my server and I am sure there will be more domains/issues coming to light.

I should also mention that I can navigate to any one of my none secure sites in a browser via https://www.domainnamehere.com and see the SSl error and the SSL cert for Site1.com

Potentially this will have more far reaching issues than just for outlook discover and I'm sure I can't be the first person to have had this issue, yet my Google Fu has failed me.

Any help here would be greatly appreciated.