As 'Send to' opens correct email client then 'Mailto' must be set up correctly. As this sends as an attachment in an email, perhaps you should use this option if the other 'Share' option fails.

Try: Click Start > Default app settings Click Email > Choose Thunderbird

Also check the Internet options and go to the Programs tab and click Set programs and make sure that Thunderbird is selected

"Try: Click Start > Default app settings Click Email > Choose Thunderbird" This is already done.

"As 'Send to' opens correct email client then 'Mailto' must be set up correctly. As this sends as an attachment in an email, perhaps you should use this option if the other 'Share' option fails."

I want to use the "share" option

"Also check the Internet options and go to the Programs tab and click Set programs and make sure that Thunderbird is selected"

don't know what you mean, more info needed

https://www.isunshare.com/windows-10/4-ways-to-open-internet-options-in-windows-10.html

Easy to access via Control Panel

that appears to be another route to "settings, default apps"

it something to do with "share" working off "charm" popup

Ok found some info : The 'Send To' feature which uses Messaging API call (Mailto) doesn’t support the built-in 'Mail' app. So to use that function you have to install an email client such as 'Thunderbird'.

Note: The “Email” button on the Ribbon does the same as Send To > Mail Recipient (needs desktop mail client).

If you do not have an email client installed there is still another way to send by using the 'Share' and built in 'Mail' app option. The built-in 'Mail' app in Windows 10 allows sending or sharing files via File Explorer by default and supports all files types.

"Ok found some info : The 'Send To' feature which uses Messaging API call (Mailto) doesn’t support the built-in 'Mail' app. So to use that function you have to install an email client such as 'Thunderbird'." Is "email client" different from the thunderbird I use to send my emails?

"Note: The “Email” button on the Ribbon does the same as Send To > Mail Recipient (needs desktop mail client)." yes.

"If you do not have an email client installed there is still another way to send by using the 'Share' and built in 'Mail' app option. The built-in 'Mail' app in Windows 10 allows sending or sharing files via File Explorer by default and supports all files types." So no chance of tagging thunderbird onto the "share" option instead of the built in "mail" option?

All information I've managed to locate says to use 'Sent to' which uses 'Mailto' protocol if wanting to use an email client of your choice or use 'Share' > using 'Mail' app if you do not have any email client installed.

If I find out anything to the contrary then I'll post info here.