We ran our own some time ago and worked closely with the state government who also hosted their own email at the time. They had Barracuda e-mail filters that they used, and they would constantly flag our emails to them as spam at random times. Of course, we could just pay Barracuda a verification fee and get the green light. They couldn't even whitelist us themselves! But those fucking spam filter appliances were everywhere at the time. It sounds like this is just another case of a shitty spam filter.