v5.2.21

Fix missed version number update

Fixed link to Security (PHPMailer#931)

Minor cleanup, fixes PHPMailer#918, broken version file

Remove duplicate check for Sender address

Remove broken check

Name constants consistently

Remove unnecessary POP3 properties
Merge branch 'master' into 6.0

# Conflicts:
#	src/PHPMailer.php
#	src/POP3.php
#	src/SMTP.php

Merge remote-tracking branch 'origin/master'

Bump version to 5.2.16

Docs tweaks

Bump version to 5.2.15

Cleanup