'550 5.1.8 Access denied, bad outbound sender' - Microsoft Community

1 of our users had account compromised. fixed unable send outgoing email. gets error: 

'550 5.1.8 access denied, bad outbound sender'

should do?

hi jamey,

in situation, suggest use admin account log into exchange admin center, , click protection> action center to check if account blocked. 

here article reference: 

removing user, domain, or ip address block list after sending spam email.

if not administrator, let global admin in organization check it.

if issue persists after unblocking office 365 account, need collect whole ndr , tenant information resolve it.

please provide tenant information , the entire ndr via the private message.

thanks,

amanda

Office / Outlook / Windows 10 / Office 365 for business