Reply email sends email to recipient instead of sender

When i sent an test email from one inbox (account) to another inbox (account), i see the email in that inbox.
tom@test.tld sending and email to maria@another-test.tld
maria receives that email.
maria is hitting the reply button on that email and sending back an answer, will actually result in
maria@another-test.tld sending an answer to maria@another-test.tld

i have to search and find the sender and email address to reply.