Email is constantly changing, and Apple recently made updates to their email service for emails hosted at the @mac.com, @me.com and @icloud.com email addresses.
Email security wasn’t well defined back in 1982, and allowed anyone to impersonate sending an email. An standard called DMARC stopped that and allows email providers to apply the most strict control. Apple chooses which servers are allowed to send emails from their domain, any other server will now be marked as spam, no exception.
Attempting to send an email from @mac.com, @me.com and @icloud.com outside of Apple Mail will immediately be marked as spam.
Apple Mail isn’t the first provider to do this, Gmail did this in 2016 and Yahoo in 2014. These changes help to limit the amount of spam from these domains and prevent phishing attacks.
If you want to send event invitations from an the hosts email address, it is now best to have them BCC their invitee list to give you the best chance to prevent you from being flagged with DMARC.
Sometimes an email will show “on behalf of domain.com” to work around this limitation with DMARC because the email address is actually written like the below address. Email clients make the format more human readable by showing the “on behalf of” separately. The resources link below has more detailed information about this and why you can’t remove it from Gmail, Yahoo, or Apple Mail.