When running Microsoft Lync client, you receive the following error:
"Either there is no default mail client or the current mail client cannot fulfill the messaging request"
The reason for this is likely that your Lync client is specifying that Outlook or Exchange should be used as the Personal Information Manager. If Outlook is not configured, Lync will not be able to attach to it to retrieve calendar / free-busy info, and may complain by popping up the message.
Open Lync, then go to Options:
Switch to the Personal tab, then in the Personal Information manager drop-down, choose "None"
Click on OK to close Lync Options, and the pop-ups should stop!