Steps to reproduce the issue

Set Joomla instance to use PHP Mail or Sendmail.
Perform some activity that causes email to be sent (registration, contact).

Expected result

Generated email is sent without issue

Actual result

Error: Could not instantiate mail function.

System information (as much as possible)

Joomla 4 or 5 on Siteground hosting

Additional comments

Some systems fail when attempting to send email which has the recipient name and email set to the same value (501 error).

libraries/src/Mail/MailTemplate.php

* @since   4.0.0
*/
 public function addRecipient($mail, $name = null, $type = 'to')
 {
     $recipient          = new \stdClass();
     $recipient->mail    = $mail;
     $recipient->name    = $name ?? $mail;    <<<<==== 
     $recipient->type    = $type;
     $this->recipients[] = $recipient;
}

addRecipient() is often called with only an email address, which causes the name to be set to the same as email.
This is causing failure on some servers (or their mail servers).

Possible workaround

$recipient->name = $name ?? strstr($mail,'@',true);

Additional preferable solution

All Joomla calls to this addRecipient() method provide an appropriate name argument.