Unable to send emails to external users, bouncing back

Usually bounced emails contain an error indicating the reason for the bounce... can you share that?

STATUS
Office 365 received the message that you specified, but couldn't deliver it to the recipient) due to the following error:

Error: 550 5.1.8 Access denied, bad outbound sender. For more information please go to http://go.microsoft.com/fwlink/?LinkId=526653. S(9333)

A non-delivery report (NDR) message was sent to .... The NDR might provide more details about why the email message wasn't delivered and how to fix the issue.

HOW TO FIX IT
Ask the sender () to follow the instructions in the NDR to fix this issue. The NDR might also include specific information for email admins. If the sender is unable to fix the issue, ask them to forward you the NDR and then follow the guidance for email admins.

Is that happening for all external email addresses?

This is O365 recommendation for that error. Basically it seems that he user is being blocked because of sending spam. This will tell you how to unblock the sender

https://support.office.com/en-us/article/fix-email-delivery-issues-for-error-code-5-1-8-in-office-365-303238b8-658d-46b6-8f45-a789acd2173b

If the user believes they haven't been sending spam themselves, it's possible that their PC has been compromised with malware. I've seen this happen when a user opened an unsafe pdf file in Adobe Reader (which has insecure settings by default) and the resulting malware seemed to be able send spam through the Offfice365 email account setup in Outlook on the computer.

That is something you will need to look into, but following the instructions will at least get the user email again.

everyone were very good...

You actually awarded the points to someone that basically gave you my answer, he just copied and pasted the info in the link I gave you. Hmmm ok