The world is moving away from SHA-1 certificates, which is a good thing from a security perspective. Major vendors like Microsoft and Google are forcing Certificate Authorities to give out certificates with the new SHA-256 hashing algorithm. They do this by simply stop trusting root CA’s which give out SHA-1 certificates after a certain date.
What can you do? Not much actually, because every new SSL certificate you purchase or renew will be SHA-256. Of course there are old operating systems which don't support SHA-256, for instance XP versions older than SP3 and Server 2003 without SP2 installed. So basically every modern OS and even much older versions support SHA-256.
Now for us Exchange guys there may be an exception, this is Windows Mobile 5 and 6 which do not support SHA-256. Yes, this operating system is very old and has been superseded by Windows Phone 7 and newer. However, it has been sold until not very long ago and products with Windows Mobile may be still available. An example of such a device is the Samsung Omnia 735.
So if you have any users with a Windows Mobile device and are planning to renew your Exchange SSL certificate, please be aware of this issue. Depending of your situation you may consider replacing the devices or even move to SHA-1 certificates which are signed by your own CA. This obviously is not a safe long-term solution but it may buy you some time before you can move to SHA-256 certificates.