After upgrading CiviCRM to 4.7.21 (on Joomla 3.7.3), no emails get sent from CiviCRM (transactional or from CiviMailing). In System Settings >> Outbound Email (SMTP/Sendmail) when I click on "Save and Send Test Email" the page keeps reloading for about a minute, and then results in 500 error ("This page isn't working").
This worked perfectly before the upgrade.
Also, I no longer see file packages/Net/SMTP.php (I re-ran the CiviCRM install just in case) which used to send email via SMTP ... is this now handled by another file in another directory? Thank you so much for any pointers.