Can not send mail in Mail

You are welcome. Did you try the Safe Mode to see if Mail worked there?

Do a backup, preferably 2 on 2 separate drives.

Quit Mail.

Go to Finder and select your user/home folder. With that Finder window as the front window, either select Finder/View/Show View options or go command - J. When the View options opens, check ’Show Library Folder’. That should make your user library folder visible in your user/home folder. Go to Library/Containers/com.apple.mail. Move the folder com.apple.mail to your desktop. You must move the entire folder, not just the contents.

Restart, re-launch Mail and test. If the problem is solved, recreate any required Mail settings and import any emails you want to save from the folder on the desktop. You can then put the folder in the Trash. If the problem remains, return the folder to where you got it replacing the one that is there.

If that doesn’t solve it repeat the above using Containers/com.apple.MailServiceAgent.

Information learned from Linc Davis. Thanks to leonie for some information contained in this.

Mail crashing

Crashes/quits unexpectedly

Not knowing what you have tried. Can you receive mail?

Mail (El Capitan): Mail troubleshooting

Mail (El Capitan): If you can’t receive messages

Mail (El Capitan): If you can’t send messages

Mail not sending or receiving messages after updating OS X

Also try the Safe Mode.

Safe Mode - About El Capitan
also Sierra

Safe Mode Startup – El Capitan. also Sierra

If it works in the Safe Mode, try running this program when booted normally and then copy and paste the output in a reply. The program was created by Etresoft, a frequent contributor. Please use copy and paste as screen shots can be hard to read. Click “Share Report” button in the toolbar, select “Copy to Clipboard” and then paste into a reply. This will show what is running on your computer. No personal information is shown.

Etrecheck – System Information

Possibly, but without more testing, it isn't certain.