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.
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).
