Outlook | Send from a Shared Mailbox
Before you are able to Send As from a shared/department email account or send on behalf of someone, you will need to have send as rights or delegate permissions.
Instructions in the Outlook Desktop Client
In Outlook click New Email
Go to Options in the top ribbon and highlight From, You can choose to show the BCC field here too.
When you open a New Email you will now see a From drop down menu above the To field.
Use the drop down arrow and choose Other Email Address.Type in the email you want. You must have Send As access in order to send emails from another account.
Next time you go to send an email it will be available to select from the drop down on the from the From field.
Video Instructions