Hi there,
I’ve heard SendGrid (the email service Bubble natively uses) can run into more spam filters than some of the other services like Postmark. I also came across a discussion here which lists a plugin to use your native email domain without a 3rd party - I haven’t used it myself so I can’t speak to it’s complexity or stability, but on paper it looks nice: Sendgrid or...? - #28 by lindsay_knowcode
And finally, related to both of these points above - sometimes email platforms will reject emails from relatively new domain names that don’t yet have much of a history (as this follows the profile of a fly-by-night spammer / fake corporation). Might be worth looking into, and if in time your URL will become known as more trustworthy.