How to avoid mail notification to external contacts?
Suppose this scenario ...
firstname.lastname@example.org is the email address used for incoming and outgoing mails.
email@example.com (odoo-user = internal contact) sends a purchase order to firstname.lastname@example.org (orders).
email@example.com (some-vendor = external contact) responds to the email (not firstname.lastname@example.org as would be expected) and Odoo still catches it at the right purchase order.
All followers of the purchase order get notified by mail about the response of email@example.com (some-vendor), meaning the following emails are sent ...
1/ firstname.lastname@example.org (with the name some-vendor) sends a mail to email@example.com (odoo-user).
2/ firstname.lastname@example.org (with the name some-vendor) sends a mail to email@example.com (orders).
The first email is very handy ... the odoo-user gets notified by mail that some-vendor replied.
The last mail is very weird because it seems as if some-vendor sends a mail internally to the orders inbox of the vendor company. People are very confused about this, even annoyed which is understandable because you should not send an email from your email address with someone else's name.
To avoid this go to Settings > Technical > Subtypes, search for Discussions and change Default to Internal only. This way the second email is not sent anymore and only internal users get notified of email responses.