I would guess that your ISP or email provider are on a dns blacklist being used by q.com.

Is there not a report explaining the return?

Armed with such evidence, you could then report the problem to your email provider and urge them to negotiate their way off the blacklist.

My ISP is Comcast (I can send email to q.com through them) but I use Thunderbird as my stand-alone email provider. So this means there is a problem with q.com and Thunderbird (Mozilla)? How do I resolve that, being as there is no way to contact Mozilla?