I use a customized e-mail and my web-master, who is also the domain owner, doesn't allow me to see the settings that the DNS provides. Some DNS services provide, beside domain name registration, services like free forwarding email. Since it is possible to set up more than one e-mail to be forwarded, I'm suspicious that my webmaster is forwarding the messages addressed to firstname.lastname@example.org to email@example.com and competing with me. I installed an e-mail tracking service called bananatag and did not predict that it would send me a lot of welcoming messages and blablas. So now I'm quite afraid that my web-master is aware that I'm tracking the emails and he'll take some countermeasures like this How can I protect against email tracking services?.
Back to my question: Is there another way to know which are the computers opening the messages addressed to my account?