Try disabling the ad blocker if any is installed and refreshing the page.
Use a browser that is Yahoo-compatible.
See if your net is working properly or not.
Clear your browser's history and cache.
If you're using Yahoo Mail on your device for the first time, you'll be asked to verify your information. Try after 12 hours if you make too many failed login attempts.
Try again after changing the password.
Check your "Drafts" & “Sent” folder to see if the email is actually sent.
Check if the recipient's email address is right or not.
Finally, request that the receiver check their spam folder, filters, and blacklisted addresses list.
Try reinstalling the app.