Federation is great when properly applied. Email is an example of federation done right. But federated social media is a mistake. Not because it is federated, but because all social media is a mistake.
SMTP by itself has no authentication and we had to add crap on top of it like SPF, DKIM, DMARC. Don't have it? Oops, sorry, to spam it goes. Oh and don't forget about a reverse DNS record for your IP address with a correct domain, otherwise some mail servers will deny you. Want other mailservers to send you emails via TLS? Well, there is DANE, but that uses DNSSEC so Gmail doesn't support it. MTA-STS exists, but it's kinda silly and was made just for people that don't want DNSSEC for some reason, but at least Gmail supports it. For some reason there is also TLS-RPT for reporting about TLS use, so mail servers will send you emails that they used TLS, yay. Want your email address to have a picture? There is BIMI, but some mail servers won't display it if you don't pay few thousand dollars a year for some special certificate. Oh and it needs to be a pretty small SVG.
Oops, your email still went to spam? Maybe your IP is on some obscure blacklist for no god damn reason? Or Outlook just randomly decided to ban your IP, but will offer you to unban it automatically by filling some weird form? Or maybe Gmail decided to randomly put your reply email to spam to someone that emailed you first?
Want to retrieve your mailbox? Oh, do you want IMAP or POP? Obviously you don't want POP, you want IMAP, but you remembered that there is this "new" standard JMAP that is saner than IMAP. Oh what's that? Nobody supports it? Oh well.
I pay Fastmail to deal with the actual hosting but I own my domain so I can go to any other provider if I want. I have never had trouble sending or receiving email. Seems to be working pretty well.
Since some clients pull images from Gravatar, it's advisable to add an image to your Gravatar account and display it instead. While not a universal solution like BIMI, this is still an acceptable option where feasible.
Never heard of clients using Gravatar, but then that's relying on a centralised service. It further proves my point that email is not a good example of "federation done right".
