Could not establish trust relationship for the SSL/TLS channel

Today I tried to add an Exchange calendar overlay to an existing calendar in SharePoint 2010 but encountered the following error:

Could not establish trust relationship for the SSL/TLS secure channel with authority ‘<mailserver>’.

I have already added the trust relationship using our OWA certificate within Central Administration (Central Administration -> Security -> Manage Trust) so figured something else was wrong.  As the error helpfully provided the correlation ID I turned to the ULS Viewer and located the following entry in the logs:

An operation failed because the following certificate has validation errors <certificate details>.  The root of the certificate chain is not a trusted root authority.

I felt this was self explanatory so to resolve the problem I also added a trust for the Root CA of our certificate issuer:

1. Open Central Administration

2. On the Security page click Manage Trust

3. Click New

4. Enter the name for the Root CA and browse to the .cer file

5. Click OK

Now the error no longer appears!