You can verify the visibility of the account using these instructions: Office 365 - Show or Hide Service Account within Global Address List. Important: In order to send as or send on behalf of an account via Outlook desktop client, the account must be visible in the Global Address List (GAL). Send as or send on-behalf of another account using Outlook for Windows.
Office 365 service accounts are hidden by default within GAL. If the account is hidden, you must make it visible. Important: In order to send-as or send-on-behalf of an account via Outlook desktop client, the account must be visible in the Global Address List (GAL). This means you can open up the mailbox and 'send as' that account. Note: If you are linked to a service account, you are automatically provided 'full mailbox' and 'send as' permissions to the account. If these have not been granted yet, please refer to the Office 365 - Manage Full Mailbox, Send As, and Send on Behalf of Permissions document. This document assumes you have been granted send as or send on behalf permissions to another account. Before you proceed, make sure you understand what send as permissions or send on behalf permissions are. There are times when you need to send a message either as a another account or on-behalf of another account.
