Dynamics 365 for Marketing is a great tool that allows marketing users to send bulk emails to contacts.
In order to target your customers and nurture them, it is a good practice to send the emails from an email address that they can recognize. A generic static email address will often decrease the chances that your email will be opened.
In this post we will explore the freemarkers (dynamic fields) capabilities present within the tool.
Here are the main points to keep in mind:
1. You can only access fields present directly on the contact's form or present on any contact's parent entity form.

You can for example set the "From address" as the email of the owner of the contact (field email present on the 'systemuser' form.
Freemarker to use in this case: {{contact.contact_systemuser_owninguser.internalemailaddress}}
But you cannot fill dynamically the sender field with the email of the owner of the parent account of the contact you are targeting.
2. You can't find the related entity to the contact in the list?
That means you need to synchronise the related entity in question with the marketing insights service.
To do that navigate to: Settings > Advanced settings > Marketing settings > Customer insights sync.

Now pick your entity and wait for a few hours before your entity will be available from within the list of related entities shown in the "Assist edit".
More infos about synchronizing entities in the link below under the section "Choose entities to sync with the marketing insights service" :
https://docs.microsoft.com/en-us/dynamics365/customer-engagement/marketing/marketing-settings