An error / warning message on my dashboard (at the top) saying - 'Send mail function may failed.'

From @kwcjr, transferred from the MainWP Facebook group:
Use below snipit to create this file in your sites Root folder; then navigate to it. You will see a white page & then receive an email if everything is working right with your PHP mail Function.

Make sure you have WP Debug ON during this test.
PLEASE delete this file after testing!