Don't get in the trap of "oh some provider wont accept my mail". Its their fault that they don't and frankly if everyone was hosting their own mail, this situation would have never happened.
I run (one of) my own mail server and send and receive mails on it regularly. At most the receiver might have to check their spam box once, but later all mails end up in inbox (this is gmail I am talking about, nobody else put that mail in spam, like hotmail/fastmail)
Its a email server I am writing myself to eventually use. I know mail-in-a-box exists but another solution providing a easy to setup reliable mail box should be useful.
Currently I have spf/dkim/dmarc setup so it can exchange with all providers and have imap for the client side.
Do you plan to open-source it? I have to agree that last time I looked into setting up my own email server, the setup procedure of pre-existing servers is what drove me away.
I imagine that if the client or an MTA can't connect to an SMTP server, then the mail would stay in an outbox queue until it is able to connect and hand it off. I don't think mail is permanently lost if an SMTP server goes down for a few hours. If an intermediary MTA gives up on connecting to an SMTP server, I would at least expect it to bounce the email back to the sender with an explanation.
I don't think email is as fragile as you seem to think.
And if deliver fails, the sender also gets an email (from their own MTA) mentioning that delivery failed.