Bounce codes › Microsoft 365
550 5.1.8: Access denied, bad outbound sender
This bounce is about the sender, not the recipient. Microsoft has blocked your own account from sending because it sent what looked like spam.
Fix your sending setup Your sending account is blocked
Microsoft blocked your account for sending too much spam, usually because it was hacked.
- Sent by: Microsoft 365 / Outlook
- Permanent (hard bounce). Sending the same message again fails again.
How Microsoft 365 words it (source):
Access denied, bad outbound sender
Why it happens
- Your password was stolen (phishing or malware) and someone is sending spam from your mailbox.
- You sent a large mailing from a normal mailbox and it tripped the outbound spam filter.
What to do
- Change the password of the account and turn on multi-factor authentication.
- Look for things you did not create: inbox rules, forwarding addresses, and messages in Sent Items.
- Ask your Microsoft 365 administrator to unblock the account. It is listed under "Restricted entities" in the Microsoft Defender portal.
- For newsletters and campaigns, use an email service made for bulk sending, not a personal mailbox.
Don't confuse it with
Every address you write to bounces with this code while the block is on. The addresses are fine: do not remove them from your list.
Questions
Is the recipient's address wrong?
No. The message never left Microsoft. Your account is blocked from sending.
How long does the block last?
Until an administrator of your organisation removes it. It does not expire by simply waiting a day.
Got a different bounce?
Paste the whole message into the bounce code lookup. It reads the code and the words and tells you whose move it is.
Related codes
550 5.2.2Submission quota exceeded (Microsoft 365)550 5.1.90Daily limit for message recipients reached (Microsoft 365)