I received two errors when trying to send an attachment over SendMail PHPMailer.
PHPMailer uses the web server's sendmail function or whatever mail server is installed. Normally there is a (default) limit to the message size. To fix this you need to:
postdrop: warning: uid=0: File too large
sendmail: fatal: root(0): message file too big
//Add this line under # SENDING MAIL
//This is bytes so will be 20.48MB
message_size_limit = 20480000
service postfix restart