Hi Francois,
Hope you are doing good.
1) Set regarding means :
a. You are tracking this email
b. Associating this email to an entity record, entity here can be account, opportunity etc
2) Tracking an email means you are creating a copy of this email into Dynamics CRM so it exists in Dynamics CRM and all associated contact/users in the email will be related to this email in CRM
3) Tracking associates email to contact/users while set regarding tracks an email and associates email to a specified entity record
4) Set regarding only associates the email to the entity record (account, opportunity etc), It doesn't associate or know the contact/users in the email with that entity record
5) While doing set regarding, If you create or attach an opportunity then the email is not tracked yet and quick create form of Opportunity doesn't have any information about the contacts/users in the email so it cannot fetch the contact information automatically
6) I understand you can save time but It gives you a choice of selecting any contact from the search instead of the contact as a sender of the mail, You also have to search an account similar to contact in the quick create form
Hope this helps.
Regards,
Mustufa F
