I didn't get the question. You are having SSL on the masking domains but not sending domains? If you are using cPanel on sending domains, it should auto-install a shared SSL otherwise you can use any free SSL service like letsencrypt.org.
Tracking domains can work with https if while adding the masking domains, you put a protocol e.g. instead of maskingdomain.com you can use https://maskingdomain.com and it should work.