Windows 10 version 1709 also known as "Creator" has been causing rather an upset recently. It appears it's breaking a number of basic functions and one of these is the ability of 3rd party applications (including some Windows functions themselves namely the "send to mail recipient)" to use the default mail program
Some of the affected programs include HP Scan to Mail, Adobe Email PDF button and larger applications like Vebra and it seems to stem from a broken MAPI process. This would indicate that any software requiring this function is affected
Here are the steps I have found to fix this issue. Note: just running "fixmapi.exe" didn't work
Make sure Outlook is set as the default Mail client
Close Outlook (use Ctrl, Alt, Delete to ensure all processors are dead)
For 32 bit versions of Outlook...
Delete MSMAPI32.DLL located C:\Program Files\Common Files\System\MSMAPI\1033\
For 64 bit versions of Outlook...
Delete MSMAPI32.DLL located C:\Program Files (x86)\Common Files\System\MSMAPI\1033\
Run fixmapi.exe (click on search and type 'fixmapi.exe'. It should appear in the list, click on it)
Open Outlook (Outlook will replace & register the Msmapi32.dll and then open).
Try the "Send To > Mail Recipient" again.
That's it.... Good Luck