We're having issues when people email us they are receiving a bounce back email. I think it is to do with our DNS settings / name server, and I really need some help. I think they'r'e wrong but I don't know where they should point to. I'd really appreciate some advice. Thanks in advance!

A website hosted by one.com with an email address connected to that domain, but I don't think the email address was registered with one.com. I didn't create it so I need help finding out where it should be.