This is a myth though (with some truth to it in certain cases). I've run my own mail infrastructure since 1999, no issues.
But most people who can run a server should be able to setup OpenSMTPd with the DKIM filter and Dovecot. It's much easier than configuring postfix like we had to do in the past.
To answer a sibling comment, the last time I received an answer is a few minutes ago. The correspondent's email infra is hosted by Google.
I used to run all the components and maintain it (even that wasn't bad), but I changed to mailu[1] about a year ago
[1] https://mailu.io
I don't know why. At the same time they don't want to get rid of the bbdd servers, or the app servers.
Maintaining a email service must not be as easy for them.
when was the last time you got a reply to an email you sent?