Even though email is supposedly "open", and federated, is no longer is really the case. Big services like Gmail are suspicious of non-big-name servers, and often flag email coming from them as spam.
About a year ago I came across an article from a guy who'd been running his own email server since the 90s, and finally gave up. I couldn't find that article in my quick search, but I did find this:
https://twitter.com/greg_1_anderson/status/1425113874722820100
"I run my own email server. It's no longer a good idea, because the anti-spam arms race makes delivery from small independent servers very difficult, even when you keep yourself off the block lists, so it's a continuous struggle. Would switch, but I have too many domains/addresses"
Thanks for posting this. It's really interesting.