Error is [0x80070005-0x000004dc-0x00000524]

My issue is similar: I run three separate email accounts in Outlook: One O365 / Exchange account and two @MSN accounts which I have had since the days of the old MSN butterfly. They have worked flawlessly this way for years, however a new installation this week netted this precise NDR error Error is [0x80070005-0x0004dc-0x000524] when sending from one of the MSN accounts, which just happens to be the one used by my back office. The inability to send from it was making my work all but impossible.

I took your detailed advice in the linked workaround posted above a step further because I am not able to make the change to my back office email in EAC as I do not have direct access to do so.

In Outlook, I selected Address Book, Tools, Options, and the "Custom" radio button from "Check lists in this order". I removed all references to "Global Address Lists", including the offending GAL from my O365 account. Et voila! Mail can be sent, and I can choose the sender from the dropdown to easily change from any mailbox to make easy work of sending back office emails directly to clients! It is literally blowing my mind that I came across this post and I truly thank you!

It's on the Ribbon in the Find group. I added this workaround as Workaround 2 in the Known Issue article, Classic Outlook error when sending from Outlook.com "You do not have permission to send on behalf of the specified user." - Microsoft Support.

Hi all,

If you are still seeing this issue, I just posted a Known Issue with a workaround until we can get a fix, Classic Outlook error when sending from Outlook.com "You do not have permission to send on behalf of the specified user." - Microsoft Support. If you are able to try the workaround, please let me know if this helps. I can now repro the issue on demand and also confirmed the workaround on my test M365 tenant.

Workaround #2 is the best option, Classic Outlook error when sending from Outlook.com "You do not have permission to send on behalf of the specified user." - Microsoft Support. Are you able to use that workaround? You can hide the Outlook.com account you are sending from the Global Address List in the M365 account. Or, another option for the time being would be to remove that Outlook.com account from the M365 Exchange Online contacts until this gets fixed.

I have not been able to resolve this and have not had any further responses from Microsoft. There also appears to be no available technical support line for Microsoft to contact on this matter...