Most email providers let you choose between two ways to get your messages. You can have a POP Post Office Protocol account that downloads all your messages to your Mac, iPhone or iPad. Or, you can have an IMAP Internet Message Access Protocol account that leaves some or all of your messages up on the mail server so you can access them from any device. For example, with a POP account, if you check email on your iPhone, the messages downloaded there will only be there; when you get back to your Mac, you won’t see them in your inbox. With IMAP, they’ll be on all your devices.