Hit error MAPISendMail Failed! General MAPI Failure [2] when try to email invoice/quotation/purchase order which outlook/thunderbird open.

This is windows security control issue. Different level of user privilege level might cause this issue. To solve it, we need to run email client as administrator level. Please follow steps below: 

1. Close your mail client. Right click on your mail client (outlook/thunderbird) in desktop and Click on Properties.

2. In the popup windows. Select 
Compatibility tab. Then select Run this program as an administrator. Click on OK button

3. Done. Please try again.