when trying to set the local-part for my custom domain email address to "~"

I get this error message:

"Only alphanumerics, periods, hyphens, and underscores are allowed in the local part of the email address, and it must begin and end with alphanumerics." all I can say is why?

"it must begin and end with alphanumerics."

this is so arbitrary! it's an email address' local-part not a username..

for many years, I have always used ~@myna.me as my email address, before and after I self-hosted my email, I use it for all of my online accounts and and all of my contacts have it, I can't change it now.

this is a spec compliant email address local-part, you don't have to read any of the specs, just read this Wikipedia article section:

https://en.wikipedia.org/wiki/Email_address#Local-part