NextDNS "Threat Intelligence Feeds" blocks this domain. It's also on a bunch of domain blocking lists (don't have a complete list). I've submitted pop.com as a false-positive on OISD.
Thank you! I really appreciate it. I wrote this elsewhere: We bought this domain a few months ago, and you're probably right, since Gmail seems to mark our outgoing emails as spam. I'm not very well-versed in how to undo the reputation the previous owners seem to have created for the domain, so if you have any recommendations, please let me know!
That's a long-term manual process. You need to go around to every spam list you can find and open up direct communication with the person/s that control it or go through their removal process (some offer a simple process, some are obnoxious, as you'd expect).
That said, I checked your site against blacklists. It appears clear -