When sending messages as another user the following errors might be received:
- Outlook in Cached Mode
- Outlook in Non-Cached Mode
To fix the issue perform the following:
- Double-check if you have Send As permissions on the mailbox you are sending from. Refer to the article How Do I Enable Send As Or Full Access Permissions On Exchange Account? for instructions.
- If permissions are correct on the server you should no longer receive the error in OWA or Outlook in Non-Cached Mode. Outlook in Cached Mode might still not work due to incorrect cached entries. Make sure that the correct address is chosen from Global Address List.