Do you still receive those pesky emails that appear to come from somebody you know but are really sent from an unknown address or domain?
Our clients don’t. If mail they receive comes from a domain other than that of the "From" address it contains, it does not go through.
It bounces off the receiving server with a 421 Syntax error that closes the transmission.
Our clients' mail, however, can be sent from a domain other than their own because we explicitly authorize this domain as a legitimate secondary source.
Our clients can use alternative mail hosts like gmail, microsoft 365 or a self-hosted alternative to send mail bearing their return address if they need to, and it is not rejected as spam.