The SSL certificate failed one or more certificate validation checks

Hi all,

For some reason this morning ActiveSync users cannot receive email - all other mail is fine. for example outlook users in the office.

The exchange connectivity analyser is showing the issue is with the SSL cert as below..

Testing the SSL certificate to make sure it's valid.
	The SSL certificate failed one or more certificate validation checks.
	Additional Details   Elapsed Time: 437 ms.
	Test Steps

The cert is self signed and has worked fine up until today. The cert has not expired.

Any help would be much appreciated.