"Delivery has failed to these recipients or groups" Error, please help!

[JTechy]

Hello all,

I'm an Admin; an end user is is being told that each time a meeting is cancelled or when receiving an email (if they do), the sender receives the error stated above.

As well as:

"There's a problem with the recipient's mailbox. Please try resending the message.."

I used the SCANPST tool to repair the ost file and still receiving the error. The end user in question has been with the company for over a decade and has old emails saved up, Outlook storage remaining is ~20GB/100GB. 

Any idea on how to solve this issue?

[#2]

Open Exchange Admin Center.
Click on Protection and select Action Center.
Check if any of the users are blocked.
Click on Unblock Account option in the description pane.
Click Yes to confirm the action.
Try sending the email and check if the error is resolved.

This May Help,
Peter