Recently I ran into an Error 26 – MAPISendMail Failed
Using a Windows 10 Pro x64 and an Outlook 2010 installation.
This Error was triggered if an external Application (FormDesigner) was trying to send an Email using the users MAPI Profile in Outlook.
1. Close Outlook
2. Locate folder :
x64 : C:\Program Files\Common Files\System\Msmapi\1033 or
x32 : C:\Program Files (x86)\Common Files\system\MSMAPI\1033
3. Rename MSMapi32.dll to MSMapi32.old
4. Open outlook again. Outlook will repair the file again.
5. Test it, the Email work again.
And the Error will be gone too