I have installed Odoo v12 on a server and I have configured an outgoing mail server given by my organization. I have tested the conection and everything seems to be ok. But for some reason the server is not sending the emails. I have tested the outgoing mail configurations on my local machine and my local odoo is sending the emails successfully, but for some reason that same configuration is not working on a server.
When I go to Settings->Technical->Emails to see the email failed, I see that the "Failure Reason" tab shows the following message "Error without exception. Probably due do sending an email without computed recipients" But I checked an the email message has all the recipients. I the next image you will see (message is in Spanish)
This is an image of the odoo log
In the next images you can see that the email server is configured
I had the same problem. Odoo answered that i should use odoo-mail-server only for transactional emails (orders and invoices) ...and that i should use an external mail-server pour mailing campaigns, to prevent them from reducing the deliverability-score of transactional emails.
I have chosen "Brevo", which does the job so far.