Steps to reproduce the issue

Verify that phpmail will send emails on a your server with Joomla 3. You can use either the test button in global configuration or create a new user in the admin or the contact form on the front end

Once you have verified that it phpmail works on your server with joomla 3 - try the same with joomla 4 on the same server

Actual result

Issue ONE

Test button in global configuration returns the following error message

The most recent request was denied because it contained an invalid security token. Please refresh the page and try again.

Issue TWO

Creating a new user in the admin returns the following error message

Save failed with the following error: Could not instantiate mail function.

This last message reveals TWO issues

1. the save was successful - it was only the sending of the mail that failed
   (in J3 there would be TWO messages - one a successs on the save and one a fail on the mail)

2. the string "Could not instantiate mail function" comes directly from phpmailer
   

   joomla-cms/libraries/vendor/phpmailer/phpmailer/src/PHPMailer.php

   Line 1968 in d797b58

   'instantiate' => 'Could not instantiate mail function.',

BUT it should be being converted to a joomla string with this code here

I suspect that fixing point 2 above will fix point 1 as well.

Issue Three

Sending a contact on the front end

As expected from the other tests it doesnt work and we get a 500 Could not instantiate mail function.