You do not have the permission to send the message on behalf of the specified user.

Question

My error code window says "You do not have the permission to send the message on behalf of the specified user."

This happens when using both the DESKTOP version of Outlook, as well as Outlook.com.

This error message gets displayed when they try to send an email address that has all permissions set to:

Read and manage permissions

Send as permissions

Send on behalf of permissions

I already tried uninstalling and then re-installing Outlook but that didn't help.

I tried changing my Outlook account's settings to not use cached exchange mode, that didn't help.

I tried to turn off and on the settings inside of Exchange email permissions window. No luck.

I clicked on the "from" button and added the "other" account name. I fear the problem may be on the Exchange server, but wanted to start here.

Answer 1

Thanks Winter.

It looks like it was the GAL. I had to go back into the User Detail Panel of each account affected and turn them back on. Then I had to wait a while until it updated.

Answer 2

Dear Terence Chase,

Good day!

Thank you for posting to Microsoft Community. We are glad to assist. After thorough research and testing, I kindly ask for the following information for reference:

1.
Consider the following scenario:

• You have Full Access and Send On Behalf Of permissions to a shared mailbox in an Exchange Server environment.
• You configure a Microsoft Outlook profile for the shared mailbox and enter your own credentials to access it.
• You send an email message from this Outlook profile.

And if you are running Outlook in Online mode, you receive the following error message:

You do not have the permission to send the message on behalf of the specified users.

In this configuration, Exchange Server requires Send As permissions to send the email message. If you don't have Send As permissions for the shared mailbox, Outlook can't send the message.
Please refer to Can't send email message when Full Access is granted to a shared mailbox - Exchange | Microsoft Learn

2.
If a mailbox is hidden from Global Address List(GAL), it can not be opened via any mail client, make sure that this isn´t cause. Please unhide it, let the GAL update and retry it.3.
If that issue happens when a user was moved from on premise to Exchange Online, check if an LegacyExchangeDN email address was added.Please refer to  "0x80070005-0x0004dc-0x000524" error when a user sends updates to Calendar items that were created before Exchange Online migration - Microsoft Support4. 
If user was typing in the "From" box, in that case always got an error. please select from the GAB.
Open and use all contacts - Microsoft Support Or,  
Delete all email addresses from the “From:” box (which is seen when you send an email). The use the “display name” from the shared box in that field to re-add it. This will force that the client check the GAL and reload the correct permissions.5.
reset the whole Outlook profile:Create an Outlook profile - Microsoft Support（Don't delete the old one) Switch to another Outlook email profile - Microsoft Support 6.
It could take some time until the internal Exchange (hold by the Information Store) process updates the content, so that the user can perform these actions. This is due to the reason that MS Exchange update content from the ActiveDirectory every two hours (if not changed). This is controlled by the “Mailbox Cache Age Limit” & “Reread Logon Quotas Interval” entries “HKLM\System\CurrentControlSet\Services\MSExchangeIS\ParametersSystem”. Additional to that time the GAL must be updated as well.I will continue to concern your status, If there are any updates, please reply to this post, I will do my best to assist you further.
Thank you for your understanding and cooperation,
I wish you all the best.

Regards,
Winter| Microsoft Community Moderator