Hello,

To send emails via sendingblue, I used Mime Email class with sendingBlue bridge,
I noticed an Exception while trying to send the email to a remote template hosted on sending blue;
Sendingblue/brevo expects to receive a templateId header to select the template to be used for this email, that is supposed to be enough;
I encountered a LogicException because an Email should always have body or an attachment,
The source of this exception comes from Email::ensureBodyValid() private method.

A simple solution for this issue, is to define an arbitrary body (I usually send the templateId as body text), so no big deal.
I do not consider this to be a bug, not a serious one at least.

That gave me the idea to make a pull request, to define a special type for Remote email, with I called RemoteEmail in the Mime component for emails hosted on the cloud, which allows the email to have an empty body, instead it will define two things, a templateId, the header name to be sent the email api,

I had to change the visibility of the class Email::ensureBodyValid() to protected instead of private, to be able to customize it's behaviour.

I included tests for RemoteEmail class in the Mime component, and Tests to integrate it with SendingBlueBridge, but I did not integration bridge tests to avoid unit tests issues in the Mailer component since the RemoteEmail class would not be recognised,

Sending Blue Test file in another branch:
https://github.com/mmouih/symfony/blob/be08c7ca6bdf44ac148f09761cdd0b9e8c755190/src/Symfony/Component/Mailer/Bridge/Sendinblue/Tests/Transport/SendinblueApiTransportRemoteEmailTest.php

Example of a failure test:

A success example below:

This RemoteEmail class is a subtitute to Email on most usages, however the body is always Empty.