Microsoft has confirmed a new issue in Classic Outlook that prevents some users from sending emails through Outlook.com. The bug is currently under investigation, but there’s no full fix yet, and the company is only offering temporary workarounds for now.
The issue shows up in a pretty straightforward way: emails never reach the recipient, and the user gets a delivery error (NDR). At the same time, incoming mail works just fine, which makes the problem harder to spot quickly.
The error itself looks like this:
“This message could not be sent. Try sending the message again later…”
And it’s accompanied by a technical code:
“You do not have the permission to send the message on behalf of the specified user.”
According to Microsoft, the issue most often affects users who have an Outlook.com account configured alongside another Exchange account within the same profile. In this setup, the client can get confused when determining the sender address.
Another trigger is when a contact exists in the system with the same SMTP address. In that case, Outlook tries to send the message “on behalf of” another user and blocks the action.
While Microsoft is working on a permanent fix, users are being offered several temporary workarounds:
Remove the Microsoft 365 Address Book from Outlook
Hide the conflicting contact from the Global Address List (GAL)
Create a new Outlook profile without extra accounts.
Switch to the new Outlook or use the web version.
These methods don’t fully resolve the issue, but they do allow users to keep working.
Interestingly, this isn’t the only problem Classic Outlook has faced recently. Microsoft has already fixed bugs related to client crashes, Gmail sync issues, and other critical errors.
Once again, this highlights a weak spot of legacy clients: they’re more complex, have more dependencies, and tend to break in less typical configurations. If Outlook is used in a corporate environment, especially with multiple accounts, issues like this can directly impact business operations. For now, the options are limited: either rely on temporary workarounds or switch to the new Outlook or web version until Microsoft rolls out a full fix.
