You can deliver email to gmail and outlook until you ... can't. Whether the IP block your mail server is on gets blacklisted, some heuristic shifts against you (domain name becomes "bad" and shifts a point score over a threshold for being spam), or some other external factor happens, your perfectly configured mail server will suddenly and possibly with no warning or sign that it's failed, fail.

For even personal mail this is pretty annoying, but if you're relying on mail for business reasons, this is completely unacceptable. You need to be able to assume that mail you send reaches your clients/customers. The chance of your private mail server getting banned might be low, but it's not low enough, and over time that chance only increases (especially if you're hosting from a server on big shared IP blocks with naughty tenets like on most major cloud providers).

IP addresses and whole address blocks may have a bad karma: if somebody ever sent spam from them, they may be marked as untrusted in various databases, and unblocking them is pretty hard. The filters prefer to err on the side of mis-marking a few legitimate self-hosted emails, instead of passing a spam salvo.

Decentralized trust is still hard.

Maybe it's a joke, but a problem I see in every "open source self hosted alternative" is that people tend to underestimate how much work is to self-host everything.

It's either paid hosting like AWS, some intermediate docker-compose solution or your own personal server machine. In every case someone has to do the gritty work. It's either a paid service, a volunteering open-source contributor, or you.

I guess try to use it as your main e-mail for a few years and you may see...