Flagging if you're lucky, they outright 550 refused my mail until I joined their sender program and applied to have my domain unblocked. Then they proceeded to gaslight me claiming my mail was never blocked even after I forwarded their own error messages and IDs back to them.
yea unfortunately I have seen those as well. It is ridiculous at times.
You got a 550? Lucky! When I worked at a non-profit that hosted our own email server, we had many instances where Microsoft would /dev/null our newsletter e-mails. Their servers would give a 250 indicating acceptance, but the e-mail was nowhere to be found (and yes, we checked the spam folder).
That's nothing compared to the joy of dealing with legit emails that are flagged as high confidence phishing.
Regular provider == (Microsoft 365 || an Exchange Server)