There was a problem opening your mailbox (after SSL cert has been installed).

can you check what you are getting with google chrome
OR
clear the browser cache on IE 8 and check

IE8 is supported upto exchange 2013

In IE to Internet Options > Advanced > in the security part untick "Do not save encrypted pages to disk"

I have tried all of the above and nothing worked sadly.

Could it be anything to do with IIS settings? I had to make a little change last night and I wonder whether that would be why? Things like unchecking a tick in the SSL or HTTP redirect settings.

So now, we can't even respond back to emails. When we hit reply, the message body is blank but no cursor for you to be able to type back. So there's evidently something wrong with IIS?

Tough to guess about, unless real links are shown.

Best way to debug is to take one of your email + decode the attachments.

My guess is the attachments may be being converted to external links (which are HTTP, rather than HTTPS).

If they attachments are actually inlined (include in the message) there should be no problem.

Fastest debugging will require you email someone an actual email, so they can debug the problem.

Did your cert require you to go to TLS 1.2? Is it enabled in your browsers? I don't think IE8 by default was set to support it.

Here are a few test sites.

https://www.ssllabs.com/ssltest/viewMyClient.html
For listing of supported protocols

https://tls1test.salesforce.com/s/
To see if TLS 1.0 was disabled

I checked TLS 1.2 and still it's got the same issue. I disabled TLS 1.0 and tried and the same thing.

So when I try to compose messages, it just doesn't open up the pop up. When I hit reply to a message, it comes up with the box but no ability to type inside. I've attached the picture of what I'm seeing.
Exchange-store.JPG

Did you document everything you did adding the new cert? Did you remove the expired cert?
Is time right on server and clients? I recently fired up a laptop that had been sitting for 6 months or so and nothing was working right until I noticed the time was about 30 minutes in the future...

Do you have a newer system to test access with just in case it is IE8 related?
If the systems are locked at IE8 are other updates more current?
Is there a corporate or at least common virus scanner on workstations?

Guys, so the bizarre solution in the end was to do the following:

No restarts helped at all in the end.

Resolution:

Install Exchange SP3 and restart the entire server. And hey presto...it worked.

Thank you for your input.