When creating a new marketing email, you must fill in the "From" lookup. However, this box is limited to a lookup of users within Dynamics. This means that if you want to send an email from a department's address (such as marketing@mycompany.com) you would need to use a license and create a D365 user for that email address. You should be able to send the email from any valid email address in your Office365 account, including from distribution list addresses and shared addresses.