There are countless reasons why emails end up in spam folders, and without more details, it’s impossible to pinpoint the exact cause.
That said, one of the most common issues is incorrect or missing DNS records, especially reverse DNS (PTR) entries. Another frequent cause is that the sending IP is listed on spam blacklists, or it belongs to an IP range flagged as “residential” (home user ASN), which many providers treat as untrustworthy.
If you don’t want to share your actual FQDN here, you could post your DNS zone file with the real domain name and IP addresses replaced by fake ones, which would allow us at least to see some of the more obvious mistakes you might have made.
It would also be helpful to see the full email headers of the messages that landed in the spam folder, as these often contain clues about why they were flagged.