No. That's not the reason. Unless you're deleting it from your iMac before you go looking for it on your phone. IMAP supports full synchronization of the server and all clients. If you delete it from one device, it will be deleted from all devices. POP3 does not support synchronization at all. By default, the first client device to download the message will delete it from the server, meaning it's not available on any other device at all (or the server).