There is a Windows PC environment 10 + Outlook 2007，After two days before the end of updating Windows Update，Outlook will occasionally jump out”Either there is no default mail client or thecurrent mail client cannot fulfill the messaging request. Please run Microsoft Outlook and set it as the default mail client.”Error message，Outlook while clicking the 'letterhead and font "、"Create Signature"、"Editor Options" will not respond。
After searching online，The problem is in”Microsoft Office desktop apps”This app，Enter "Settings" - "Applications"，After remove both the boot，The problem can be solved。
[Links]
- Outlook 2010 signature can not edit window – Microsoft Community
