@Steve thank you so much for this link! That confirmed my guess, that the MX record is set to hostB.

@StephenOstermiller Thank you for your comment! I checked the MX records on domainA but it is pointing to hostA. But I found out, that the previous web developer already transferred domainA to another registrar. I will check the MX records there when I got the log in credential of the new registrar. I think, I will find my answers there. :)