Office 365 — Sending from Shared Mailboxes
Microsoft Outlook using Microsoft Exchange or Outlook 365 can support multiple mailboxes but by default will send from the user's primary mailbox.
To send a message from a shared mailbox:
- Start a new message.
- Choose Options → Show From.
- Click From. Enter the address you wish to send from, or click From to select from the Global Address List.
This only needs to be done once. In future the shared mailbox can be selected more easily using the From button drop-down list.