On 1/1/2017 after a brief run of Zen Cart v1.5.5C, 1.5.5D was released almost directly on its heels. This release looks to be a catch up of bugs and patches for the 1.5.5 line. Bugs and patches such as the PHPMailer security patch (Dec 2016), PayPal deleting the comma from PayPal Standard from Jan 2017 and the 3/2016 Trustwave Security report Patch TWSL2016-006 are all included!