It still possible to send mail from a small mail server, but there no grantee at all that messages will not end up in a spam folder. Most likely Gmail uses some ML based spam filter which has relatively high rate of false positives. For big senders they can do something manually, but they don't care about smaller ones. Gmail is too big and have little incentives to improve they spam filter (until users will start to leave, which will not happen anytime soon).