WP notification emails not working

I have an issue related to email notifications since I moved to a new Brazilian host due to difficulties of host support. Nowadays, I joined Locaweb and it’s a good host in Brazil.

Every time I need to upgrade WP, WP stops sending notification emails. The only way to make it work, is installing the plugin WP-MAIL-SMTP, creating a specific email account for this purpose (the same email found at the email “wp-admin > settings > general > email”) and making a modification to the file “/public_html/wp-includes/class-phpmailer.php” as described below:

protected function smtpSend($header, $body) {
(...)

     if ('' == $this->Sender) {
          $smtp_from = $this->From;
     } else {
          $smtp_from = $this->Sender;
     }

     //Begin Modification - Added line of code

     //Overwriting smtp_from with the same email found at
     //"wp-admin > settings > general > email"

     $smtp_from = "[email protected]

     //End Modification - Added line of code

(...)

Note that, PHP function mail() is activated on the host and working as well.