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.