We’ve all seen it.  The little “On My Mac” label in Apple Mail, or Entourage’s “On My Computer” label…

Almost every modern mail package offers a similar feature, but many people don’t understand what this means…  So here’s the scoop.

There are three different major ways to handle email:

• Post Office Protocol (POP 3)
• IMAP

POP, or Post Office Protocol is an older standard from 1984.  The core issue with POP is it is designed to remove your mail  from the server and it is only stored on your local machine.

Pros:

• • Your Mail is stored locally so the server does not have to have much storage or processing power, excellent for old hardware.
  • Supported by just about every mail package on the market due to it’s age

Cons:

• • Your Mail is stored locally, so you can not share mail between mail clients, without duplicating each mail message.
  • Does not support folder access on the Server (all folders are local to your machine).
  • No multipart messages, or Flag Support

IMAP, or “Internet Message Access Protocol”, was designed in 1988 to fill in gaps that Pop created.  For example, the major issue with pop is that you are unable to share basic information between different mail applications.  So IMAP allows the server to track if a message has been read, deleted, forwarded, etc, so that all IMAP clients are aware of this.

Simply stated if you check your mail from home, when you get to work, your work mail will be read as well.  Anything you delete from your work computer, will also be deleted from your home computer.  You also can have folders that are on the server so you can organize your email, and depending on the installation IMAP supports server based searches.

Pros:

• • Tracks the state of your email, and your mail clients can use this information to keep themselves all in sync.
  • Supported by almost all major email packages
  • More robust, and you can have multiple email programs using the same email account simulateously.
  • Folders can be created, and stored on the server, allowing all applications to access those mail messages.

Cons:

• • Your mail is stored on the server, but most email packages cache the mail locally to help reduce any lag…  As well as granting off-line access to your mail.
  • More complex then POP, and slightly slower, depending on the mail application used.
  • Has a larger “disk storage” requirement than POP3.

So, in Apple Mail, you will see the Server name, and the “Inbox”.  If you have an IMAP connection, you can place folders on the server and organize your mail on the server.  The “On My Mac” or “On My Computer” refer to folders / mailboxes that have been created on your local computer and stored there. If you have a computer failure, and don’t have backups, that mail will be lost with no way to recover it.

Now, Pop3 does have unofficial support for “Keeping your mail on the server for XXX days”.  But this is not a standard and how it works will vary widely  depending on your mail applications.  For example, some applications only allow 7 days, or up to 30…  But even so, your read / delete and reply status will not be stored with the mail.  So if another mail package connects and downloads the mail it will appear to be unread.

IMAP is currently the better choice, assuming it’s available.  In most cases your mail provider should support IMAP connections…