I set up a Custom Process to send a bulk email. The emails were flagged as spam, some bounced back and my email was blocked from sending any emails.
This has something to do with your Microsoft account.
Refer to this Microsoft article Remove blocked users from the Restricted users portal in Microsoft 365,
"If a user exceeds one of the outbound sending limits as specified in the service limits or in outbound spam policies, the user is restricted from sending email, but they can still receive email.
Contact your IT administrator to have this checked