Dear Odoo community,

I have trouble to understand how the email marketing sending process works. (note that I am not a developer)

We use Odoo 12 on a dedicated private server. 

Our email outgoing provider is SendGrid:
- SMTP Server smtp.sendgrid.net 
- SMTP Port 465
- Connection Security SSL/TLS

Our problem: our last two email campaigns have failed to be sent to all our recipients.

Here are the stats of our last one:

12 037 recipients

In my campaign (email marketing app) I have these numbers:
- 6 294 emails sent (about 50% success)
- 1 149 email ignored (this is normal; basically all the email in double or blacklisted are ignored, and by the nature of our business a lot are duplicated) 

In ‘Technical > Email’ => group by status on our campaign
- 438 Emails cancelled
- 4 594 Emails Outgoing

In 'Technical > Tracking email'
- 6307 email sent or opened

You can notice the numbers don't add up... but that is not my biggest concerns, as my goal is to have no emails in Canceled or Outgoing status. Does someone as any clue?

Note: I know we can resent the emails in Canceled or Outgoing status, but this process is way too long (we tried an action server, but it takes ages just for 200 emails to be sent).

Note 2: does anyone know where to find some documentation about Email Marketing App? (sending process, Templates, etc).

Thank you in advance for your help!