Send message button missing from Mail app's compose window I have an older iMac running macOS High Sierra. I use the Apple's Mail app version 11.5 for handling my emails. Something has changed in my system recently such that the Send message button has gone missing from the compose window toolbar. The compose window used to have a Send message icon for me to send my messages. Now it is missing.

This first figure below shows the toolbar in the Mail app.



Now if I click the New Message button I get the following screen:



And here is the expanded toolbar in the New Message window.



As you can see there is no way to send the message. I've tried starting in safe mode but to no avail. I went to Mail app Preferences and everything appears to be in order. I've tried all 3 accounts, but with the same result.  Does anyone know what's happened, or can help me in any way?
Most likely explanation is that the Send message button accidentally got removed from the New Message window.

Open Mail app and open a new message window. Click on View → Customize Toolbar... command in the Menu bar. Now drag either the Send button, or the entire default set into the toolbar area to restore the Send message button in the compose window.