On Wednesday, December 15th, a large percentage of Gmail users suffered an outage that caused incorrect "Bounce back" emails. These bounce back messages were erroneously informing the senders that the selected email address did not exist when in fact it did.
Paragon has protections in place so when they receive an error message of this type, the address is opted out automatically from future emails. Due to this Gmail outage many recipients were automatically opted out of emails from Paragon.
Paragon is researching ways to correct the affected accounts en mass however any user or contact that is opted out has the option to utilize the standard Opt-in procedure.