You must first set the “Send As” permissions on the server.
Open Outlook, and on the top toolbar, click Outlook, then Preferences.
On the preferences pane, click on Accounts to add a new account.
Down on the bottom left of your Accounts window, click the + Symbol to add a new account. You want to choose E-Mail
When the new email box comes up, you want to enter the email address of the shared mailbox (or whatever email address you will be sending as), then just click in the password box, but do not put anything in here. When you do that, the rest of the window will open.
Now that the rest of the box opened up, we can fill out the rest of the form. For the Username, you want to put in your real email address. This is the one that will be used for authentication. Next, you want to make sure that IMAP is the Type you are using, for the incoming server, just put a . in the box. For the outgoing server, you want to use smtp.office365.com. You want to check the first box to override the Port, and make sure you change the port to 587. Then you want to check the box that says “Use SSL to connect“
next, you want to click “Add Account”
Once you add the account, you will see some new fields we have to fill out to make this work. First, you might want to change your “Full Name“. This will be the display name used when you send an email from the shared mailbox. You then want to verify that the
“E-Mail address” is the correct email address of the Shared Mailbox.
Next, for the Username & Password fields, you want to use your real account for authentication. – Type your email and password.
Next, click More Options
On this next screen, you want to use the drop-down and select User Name and Password. Then put in your User name and password to your real account. This is what is used to log on to the Shared Mailbox.
You do not need to put anything in the Unqualified domain box. Hit OK when finished.
Once you hit OK, it will take you back to your accounts page. It should look something like this. Just verify the settings and close out that window.
You can now go and create a new email. You should have a From dropdown. You need to choose your new Shared Mailbox to be the From address. You should now be able to send as that shared mailbox.