Emails on iMac

Any ideas why one particular email sender's emails come straight through to the iMac only when it is on - otherwise they come to the iPad or iPhone. I have deleted the incoming mail box but each time that sender emails the box is reactivated. All other emails are received/shown on my iPad and iPhone.... This only happens when the iMac is on - otherwise the emails just come through, as all others, on the iPad or iPhone. Thank You!