RE: Using workflows to send Marketing email
Hi Partner,
It is difficult to send marketing email with workflow, because currently there isn't open process or action in workflow to send marketing email directly.
However, you can copy HTML output of the marketing email and use the output code in "Send an email(V2)" action of flow to send email.

Full process:
1. Create your custom channel tile.
https://docs.microsoft.com/en-us/dynamics365/marketing/developer/extend-customer-journeys-custom-channels
2. Create a flow on creation of custom channel activity, due to the link to corresponding contact will be included in the custom channel activity record,(a look up field, called Contact)
therefore, we can use data of that field to retrieve information of the parent contact, and it is also possible to show email content dynamically.
Action 1 and 2:

Action 3:


Result:

Like the workflow tile and other tiles, the number of processed contacts will also be displayed above the custom tile.
You can refer to Dilyanas's tutorial to build a custom tile if you thought documentation was not easy to understand.
https://community.dynamics.com/365/marketing/b/dynamics-365-for-marketing-community-blogs/posts/custom-touch-point-custom-tile-in-customer-journey-in-less-than-10-minutes
In a word, it can be said that we convert marketing email to activity email, as what my last screenshot revealed, the email is sent from our CRM account instead of domain of marketing email.
Regards,
Clofly