I’ve been having this same issue for years. I have a personal website and use my personal gmail account that I’ve been using for nearly fifteen years. All replies i send to customers that contact me via the form submission on my website go directly to spam. I have yet to find a fix. I hear many people say “this is an email issue” and not a Squarespace issue, but it ONLY happen with clients who contact me via the squarespace form. It’s absolutely costing me work and I would love to find an actual solution.