with windows 8.* claws must be in the registered applications to be usable as a mailto:protocol handler in the system "default apps" settings, it seem to work fine with the following register changes: Windows Registry Editor Version 5.00 [HKEY_LOCAL_MACHINE\SOFTWARE\Classes\Claws-Mail.Url.mailto] [HKEY_LOCAL_MACHINE\SOFTWARE\Classes\Claws-Mail.Url.mailto\shell] [HKEY_LOCAL_MACHINE\SOFTWARE\Classes\Claws-Mail.Url.mailto\shell\open] [HKEY_LOCAL_MACHINE\SOFTWARE\Classes\Claws-Mail.Url.mailto\shell\open\command] @="C:\\Program Files (x86)\\Claws Mail\\claws-mail.exe --compose %1" [HKEY_LOCAL_MACHINE\SOFTWARE\Clients\Mail\Claws Mail\Capabilities] "ApplicationDescription"="Claws email client" "ApplicationIcon"="C:\Program Files (x86)\Claws Mail\claws-mail.exe,0" "ApplicationName"="Claws Mail" [HKEY_LOCAL_MACHINE\SOFTWARE\Clients\Mail\Claws Mail\Capabilities\URLAssociations] "mailto"="Claws-Mail.Url.mailto" [HKEY_LOCAL_MACHINE\SOFTWARE\RegisteredApplications] "Claws Mail"="Software\\Clients\\Mail\\Claws Mail\\Capabilities" hope this help.
This will be fixed in upcoming release, thanks.