Added functionality such that an administrator can type
sbin/guard test xxx@open-xchange.com
which will result in guard checking for the existance of the email xxx@open-xchange.com on the server, and display the response.

Can also be used to verify proper configuration of a custom mail Resolver

There is no core change to Guard functionality. Just calls the mail resolver, displays result, and exits

commit 513e67cbe31fc55475bb1ca49c96c64a2b95fa77