Why Ghost admin invitations are sent to some emails but not others?

Hello, I am using ghost instance 2.25.8 on my server. When adding an admin which has a gmail or hotmail the invitation is sent perfectly. But with some custom domains (e.g. myrealdomain.com) I get an error message from Ghost:

Error sending email! Error sending email: Failed to send email. Reason: Can't send mail - all recipients were rejected. Please check your email settings and resend the invitation.

Do you think this error is caused by the recipient server rejecting the email? For example by the spam filter?

Yeah I would think so - what mail config are you using? You could try a few different methods to see if they have better deliverablity