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 -
One trick for upping your reputation with Gmail is to have as many of those recipients as possible (who need to dig into their spam folder sometimes) click reply and write back to you naturally. Gmail considers that behavior to be highly indicative of "not spam" well beyond simply using the "not spam" button.
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.