I was having this same issue and I found what was wrong. 

Odoo tracks all emails that are created under Settings -> Technical -> Emails and every hour, sends all the emails at one time. However, our email service (AWS) only allowed 14 emails/second. This meant that the 15th email in the queue didn't get sent, because it was sending them all at once. Our solution was to increase the frequency of the Scheduled Action (Mail: Email Queue Manager) that sends the emails to every 15 mins.

We have almost the same problem: the emails are delivered only to a part of our customers, all the rest is never delivered. And this behavior never changes, because someone who gets an email once, will always get emails and those who do not get an email, will never get one
We tried also to send emails to our own email addresses and the same problem appeared: some of us get the emails, other never get them! Also we exchanged our email addresses, in order to find out what causes the problems. The email address of the "faulty partner" has been switched to a "proper working" partner. After this exchange the "proper working" partner does not get the emails anymore. So it is definitively the email address that is causing this problem. It is obvious that all settings in Odoo have been checked several times, over and over again, but no wrong setting there. So, what can we do more?
Many thanks for your help!
Sven

Hi,

There are different things, you need to check.

1) Email Server Configuration: You need to check this first. But you have done it already as per your post.

2) Email status in Setting/mails menu you need to check whether the email status is shown as delivered or failed. That is the log for you.

Based on the email status, Check the following

1) If it is not delivered for anybody:

   a) Check the email id of the customer

   b) Check the email server configuration and restrictions in proxy or gateway.

2) If not delivered for some people:

   a) Check the email id and try to send it from other email clients(outlook)

   b) b) Check the email server configuration and restrictions in proxy or gateway.

Hope it helps.

thanks

we have encountered the same problem.

After removing the preferred outgoing mail server, everything worked fine again.

First we had indicated which mail server to use for outgoing offers and invoices.

Apparently this was the problem.

If you are using gmail and configure as incoming mail  first go to gmail

gmail  setting/security/lesssecureapp 

make it turn on first

and configure incoming mail as smtp.gmail,com    

Please change the connection security as TLS(STARTTLS) instead of SSL/TLS and check. Even though SMTP port for this is 25 by default, you may have to verify from your service provider and use the correct port number.

I have similar problem but the emails are always recognized as spam by my Outlook if sent to my email. the attachment are hidden. It can be really annoying for some customers.

What could I do ?

Check the email template defined in the Settings->Technical->Email->Templates and make sure the configuration is correct. You can edit this section and the edit will not be reverted when server restarts as email templates are defined in the data tag with an attribute noupdate="1". Your problem will be in retrieving values for the fields in email template. If possible provide your email templates's xml code.

Hello people,

We recognized this problem in our situation was the problem that we had two different SMTP servers with the same priority. We've change these and it works!

I hope it work for you to!