I have had my own e-mail server since 2017, and there is only one instance where I know I had a delivery problem, and in that case it was funny, because I wrote to two people who had the same e-mail server, and one got the e-mail the other did not. So probably a SPAM issue, but certainly nothing my e-mail server could have done better...

AFAIK, that case in 2019 was the only time an e-mail went missing.