There has been no alert that there was an answer for this message, thus answering only now. 

I do not know what do you mean by "retrying to send mails using the same user with who is supposed to send the mail"?

There was a colleague who said that it's better we change the sender email to have the same domain than the erp's domain. So, we made another email address only for the purpose of sending emails from the erp. And the emails do leave, if I go and retry but otherwise they always fail. I do not change any email address when retrying, just click the button to retry and it goes. But I can't be there like 24 hour per day checking if I need to retry the email queue.

Of course, I'm really ready to stop using the whole Odoo at this point but there has been enormous effort otherwise put into the system, so I would be willing to give a chance for it. I had paid consultants also looking at it and the answer was basically that Odoo's system is problematic, and no solution. I could imagine anyhow that some of us are using it successfully, right? I try to think in a positive way and things should have a solution. :)