To fix this problem I started outlook from the command line with the parameter /safe:
"C:\Program Files\Microsoft Office\root\Office16\OUTLOOK.EXE" /safe
Solved “Either there is no default mail client or the current mail client cannot fulfill the messaging request” when starting Microsoft Office Communicator 2007 R2
I got an error when starting Microsoft Office Communicator 2007 R2 on a Windows 2008 R2 machine with Microsoft Office 2010 installed: “Either there is no default mail client or the current mail client cannot fulfill the messaging request”.
Solution
This error was resolved by installing the following update: Microsoft Office Communicator 2007 R2 Hotfix KB 2028888