Unable to send emails

Question

I am unable to send emails and getting the error message..sending reported error (0x800ccc78) Cannot send the message. Verify the email address in your account properties. The server responded 550 This account has been disabled due to suspicious activity.

How do I fix?

TIA

Answer 1

Hello Timmy Houston

I understand how disruptive it is to receive error code ⁠0x800ccc78⁠ with a ⁠550⁠ server response stating your account has been disabled due to suspicious activity.

Have you already tried Q&A Assist suggestions? In addition, could you please confirm:

• What is the domain or provider of the email address giving you this error? (For example: Is it a Microsoft address like ⁠@outlook.com⁠/⁠@hotmail.com⁠, or a third-party account like ⁠@gmail.com⁠, ⁠@yahoo.com⁠, or one from your local internet provider?)

In the meantime, please try the following steps if you are using a personal Microsoft account:

1, Verify and unlock your account online: go to outlook.live.com and sign in there.

The web interface will immediately challenge you with a security checkpoint or an SMS verification to prove you are the real owner and unlock the block.

2, Update and secure your password

Because the server detected "suspicious activity," your current password may have been exposed.

• Change your password immediately via https://account.live.com/ResetPassword.aspx
• You can also check your recent login history at https://account.live.com/Activity to see if there were unauthorized access attempts.

3, Repair the Outlook desktop profile

Once your account is unlocked and working on the web browser, return to Classic Outlook.

Go to “File” > “Account Settings” > “Account Settings”, select your email address, click “Repair”, and update it with your new password to clear out the cached error code.

Please feel free to let me know how it goes after trying these steps. Thank you for your understanding.

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Answer 2

The error indicates the sending account has been disabled due to suspicious activity (likely flagged for spam or compromise). Outlook cannot fix this locally; the mailbox must be unblocked and secured on the service side.

Follow these steps:

1. Check if the account is compromised
   • Review recent sign-in activity and any unusual sent items, forwarding rules, or auto‑replies.
   • Use the guidance in Determine whether your account is compromised and Secure a compromised email account in Exchange Online to reset the password and remove any malicious rules or delegates. See: Secure a compromised email account in Exchange Online.
2. Ask the email administrator to unblock the account
   • The mailbox has been restricted from sending. An email admin must:
     • Go to the Restricted entities page in the Microsoft 365 Defender portal.
     • Unblock the user account.
   • After unblocking, sending restrictions are usually removed within about an hour.
   • Admin steps are described in Remove blocked users from the Restricted entities page.
3. If this is a personal Microsoft 365/Outlook.com account
   • Use the account security and recovery guidance to secure the account and then contact Microsoft support if sending is still blocked after securing it.
   • Follow the steps in How to recover a hacked or compromised Microsoft account and then use the official support/contact options if needed.
4. Try sending again from Outlook
   • After the account is unblocked and secured, restart Outlook and send a test message.

To reduce the chance of future blocks, follow the recommendations in Top 10 ways to secure your business data (strong authentication, avoiding password reuse, etc.).

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References:

• NDR error code "550 5.1.8 Access denied, bad outbound sender"
• NDR error codes 550 5.7.703, 550 5.7.705, 550 5.7.708, and 550 5.7.750
• How do I send an email to Microsoft about my account being hacked and taken over by <removed> ? - Microsoft Q&A