If you notice that you have started to receive this type of bounce-back message, it means that your provider, Office 365, has blocked your sending.
"Your message couldn't be delivered because you weren't recognized as a valid sender. The most common reason for this is that your email address is suspected of sending spam and it's no longer allowed to send messages outside of your organization."
Office 365 typically blocks accounts for activity that it considers suspicious.
Here are some examples of suspicious activity:
Unusual sending volumes (a sudden increase in sending volume)
Recipients marking emails as spam
Your emails were categorized as spam
Here’s what you can do to restore your sending process:
Unblock Your Account in Office Admin centre:
Log in to your Office tenant under Admin account and go directly to the Restricted Users page: https://security.microsoft.com/restrictedusers
You will see a list of restricted email accounts
Select an account and click "Unblock"
You may also contact Office support to clarify why your account was blocked.
Check Your Sending Volumes in Reply.
User a Ramp up mode to automatically increase your volumes.
Also, adjust email delays so you do not send too quickly.
Go to Settings (general) -> Emails accounts -> select an account - Safety
Here is an example of such setup:
How to Resume Sending in Reply and re-send Emails to Bounced Prospects
When you have unblocked your email account and are ready to send, you can resume your sending process manually from the "Reply" option.
The system will automatically re-send the bounced steps when your email account is resumed in Reply.
You can find more details about these features here:
Alternatively, you can resume them manually.
To find such prospects, use the filter "Bounce: sender's provider block"
Select your prospects and remove a Bounced status:
Click "Yes" when the system asks you to re-send the bounced step to the prospect:
