Expired SSL Certificate

My ISP let a SSL certificate expire. This was renewed but Mail (v11.5) only sees the old certificate. When I checked the email server address with Digicert's SSL Installation Diagnostics Tool this shows the new certificate being in place. (See screenshot). I also tried to set another email client up on the Mac (Postbox) but this would also only see the old expired certificate. I tried setting up an iPad with the same email account and that would also only see the old certificate. Tried a Windows laptop and that would see the new certificate. I am not sure if the problem is my end or theirs. Does anyone have any suggestions?

Also, would the COMODO RSA Domain Validation Secure Server CA also have a red cross against it?

Thanks for any help.