The mail transport (aka Swift_Transport_MailTransport) in Swift Mailer before 5.4.5 might allow remote attackers to pass extra parameters to the mail command and consequently execute arbitrary code via a \" (backslash double quote) in a crafted e-mail address in the (1) From, (2) ReturnPath, or (3) Sender header.
https://www.exploit-db.com/exploits/42221/
https://www.exploit-db.com/exploits/40986/
https://github.com/swiftmailer/swiftmailer/blob/5.x/CHANGES