Cannot send email from CiviCRM after upgrade to 4.7.21 (Joomla 3.7.3)
As Mark said this is a known regression and we are looking at an expedited release. However as i can feel even a release this coming US Wednesday may be a bit late. 

What i would advise you to do is to if you cannot wait make a patch to the file vendor/pear/net_smtp/Net/SMTP.php along the lines of https://github.com/civicrm/civicrm-core/pull/10622/files. The relevant block of code will be around ~line 600 of that particular file. 

Otherwise wait until 4.7.22 comes out which will be very soon. In regards to why this may not affect all users, it only affects users if they need to authenticate to the SMTP server over TLS