554 6.6.0 Error sending message for delivery.

Question

554 6.6.0 Error sending message for delivery.

I get this response every time I try to send a new message from Outlook.

I can forward messages, and I can send from Yahoo, but not from Outlook.

Answer 1

Hello,

I don't think that's the entire error message, can you share the complete version of the error, and edit to mask your email address to protect your privacy?

Also, what if you send from www.outlook.com, does it work?

Answer 2

Thanks for your suggestions.

The error reads

From: System Administrator

Subject: Undeliverable: Trying again

Your message did not reach some or all of the intended recipients.

Subject: Trying again

Sent 11/11/2025 12:32

The following recipient(s) cannot be reached:

W******* C on 11/11/2025 12:32

554 6.6.0 Error sending message for delivery.

I sometimes get this when trying to send, despite being a short test message with no attachments or special features:
This message seems suspicious...

It could be from a legitimate sender, but there’s a chance it’s an attempt by a hacker to gain access to your system. Tap below to see a list of actions you can take.

Trying to send from Outlook.com reached one recipient, but the cc to me did not arrive.

Answer 3

From Outlook.com I got:

I'm afraid I wasn't able to deliver the following message. This is a permanent error; I've given up. Sorry it didn't work out. Subject: Trying Outlook.com

Answer 4

Can you try switching your email to plain text, send via outlook.com, and see if it works

https://support.microsoft.com/en-us/office/change-the-message-format-to-html-rich-text-format-or-plain-text-in-outlook-338a389d-11da-47fe-b693-cf41f792fefa

Answer 5

I have the same issue for the past couple of days. I get "554 6.6.0 Error sending message for delivery". I even tried to send it to myself but same error message. I am sending from Outlook, Office Pro 2019. It doesn't matter if I'm sending to just 1 person or 10 people with no attachment, it still will not go.