Introduction
This article describes:
- A possible reason that you cannot send email from Exchange Online
- A possible reason that you cannot send email to an external (non-vt.edu) account
- The reason you get the following email bounce-back errors
- "550 5.1.8 Access denied" error, and/or
- "Your message couldn't be delivered because you weren't recognized as a valid sender".
Instructions
Your message couldn't be delivered because you weren't recognized as a valid sender. If someone continuously sends emails that are classified as spam from Exchange Online, generally to external addresses (non-vt.edu), they can be added to the Restricted Users list which will restrict them from sending email. However, they will still be able to receive it. They will be listed in the service as a bad outbound sender.
Microsoft has a limit on the number of emails a Microsoft 365 account can send per day (see Exchange Online limits for these limits). When a Microsoft 365 account starts to exceed this limit, Microsoft's spam algorithms can consider the account to be a spammer. At this point, the account is put on the block list.
The account holder will receive a Non-Delivery Report (NDR) that states:
Delivery has failed to these recipients or groups:
[USER@DOMAIN]
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 email. Contact your email admin for assistance.
Diagnostic information for administrators:
Generating server: X.Y.prod.outlook.com [USER@DOMAIN.COM]
Remote Server returned '550 5.1.8 Access denied, bad outbound sender AS(42003)'
Image of Error Message
Resolution
Virginia Tech has no control over these limits or the decision to block the account.
The resolution to this problem involves several aspects:
- Use this Service Catalog entry to request removal from the restricted users list. Upon removal, it can take several hours to be able to send email again.
- Do not send mass-mailings with either the Virginia Tech Exchange Online or Gmail mail services, both are highly sensitive to potential spamming and could potentially lock mailboxes for as little as 100 recipients. To send to large groups, use Google Groups or O365 Groups as the messaging mechanism.
- Discontinue the large sending that resulted in the initial Restricted Users blockage, otherwise the account will likely be re-blocked after the restriction is lifted.
- Reword your message so it will not appear as spam. This knowledgebase article might help.