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POP |
IMAP |
| Mail is primarily stored... |
On the computer where you check your mail. Incoming messages are stored on the server until your e-mail program checks for new mail. Messages are then downloaded to your machine, and deleted from the server (depending on your settings). |
On the server. When you check your mail your e-mail program first downloads a list of messages that are in the inbox, along with a small amount of information like who the message is from and its subject (this is the header of a message). Generally when you open a message you are requesting that your e-mail program downloads the entire message, including any attachments, but leaves the message on the server until you delete it. Some mail clients are configured to download the entire message instead of downloading the header and rest of the message separately. |
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| Mail folders on the server that are accessible... |
Generally only the Inbox. If you have separate folders to store your mail, they generally reside on your computer, not on the server (which means they are only accessible from the computer that they are stored on). |
Full access to mail folders on the server from any location or computer when using IMAP. |
| Access to your mail when you are not connected to the internet (or when the mail server is down)... |
Any messages (or attachments) that have been downloaded from the server are fully accessible when you are offline. |
When checking mail via IMAP, in most cases a list of messages is downloaded first. Contents of the messages (or included attachments) are not accessible offline until they have been downloaded from the server (which generally happens when you open the message for viewing). In some clients (where messages are not entirely downloaded automatically) you can mark which folders you want to be able to work with offline, then when you disconnect, the contents are downloaded in their entirety. When you reconnect, any changes you may have made are synchronized. |
| Connections to the mail server occur... |
Generally on a timed interval. Mail programs can be configured to check mail as frequently or infrequently as you would like. Once the mail program has made a connection, it downloads all new messages to the machine that you are currently on, then disconnects until the next time it is scheduled to check for new mail messages. |
The number of connections varies from client to client, but, in general, IMAP is less intensive on the e-mail server than POP. Most clients keep one or two connections open to the server at a time. They will stay open until one end or the other closes the connection. These connections are re-used regularly. Some clients keep only one connection, some keep one for the INBOX and another for all other folders. |
| Messages are deleted from the server... |
Messages are usually deleted from the server once new mail has successfully been downloaded, leaving the mailbox on the server empty. Most POP e-mail programs can be configured to leave the mail on the server for a period of time (seven days, for example) or to leave mail on the server indefinitely. If your program is configured to leave some mail on the server, when you delete mail from your inbox on your computer, you are generally not deleting it off the server as well. |
First, messages are marked for deletion. Messages that are marked for deletion can be hidden from sight, moved to a "trash" folder, or may remain visible in your inbox with some sort of indication that it is marked for deletion (for example, a line drawn through it, or a red dot next to the message). The messages are not deleted until the inbox is "purged" or "expunged" (which means all messages marked for deletion actually are deleted). |
| Backups... |
Since messages are primarily stored on your computer, it is possible for you to backup your mail into disks or other media. |
Messages are stored on the server, so theoretically if there was a catastrophic server crash your messages could be lost. However there is a much greater chance of your computer causing data loss though crashing or configuration problems than the mail server experiencing problems like this . Most mail servers around campus are backed up more often than the average workstation. Still, it is possible (and highly recommended) that you copy your messages from your IMAP mail account to folders on your local machine to back them up. |
| Checking mail from multiple locations... |
If your e-mail program is set to delete email after it's been downloaded, then the computer you use to read your email is the only place that email exists. People who travel, or who use several computers to read their email, may have problems with this aspect of POP. They can only get to new messages from the road, or, if they leave mail on the server, they must remember to go back and delete messages from the server later and messages may be downloaded multiple times leaving duplicate messages in your inbox. Accessing mail that was downloaded to one machine and deleted from the server is very difficult if not impossible. |
You can set up your account so that your e-mail and folders are the same wherever you use e-mail (in sites such as a public lab, a dorm room, a summer address, an office, etc.) because you will be running e-mail off of the same server during each e-mail session. You can also use webmail from any machine with an internet browser and have access to all your mail on the server. |
| Control over which messages are downloaded from the server... |
You have no control over which messages are downloaded from the server to your machine. If it is new mail, it's downloaded regardless. |
In most clients you decide which messages are downloaded from the server by choosing to open them. |
| Disk Quotas |
If the POP email program is set to delete all messages from the server after it downloads them, then the chances of going over quota are minimal. It is still possible, however, to occasionally receive enough mail to put the account over quota before the POP e-mail client has a chance to download and delete the messages from the server.
But if the client is set to leave the messages on the email server, email continues to pile up without your knowledge. When you read your email on your local computer and delete it, you may assume it's gone altogether, but actually it's still there on the server, taking up space. After a POP email program deletes a message, it deletes only the local copy. Eventually you could build up enough mail to put you over your quota, thus causing any new mail to be returned to the sender without you ever seeing it. |
Disk quotas are problematic if you want to save a lot of messages. You may run out of disk space on the server, especially if you keep messages with large attachments. Eventually you could build up enough mail to put you over your quota, which may cause any new mail to be returned to the sender without you ever seeing it. However, you can copy mail to your computer, then delete it from the server to save server space while maintaining a large archive of mail. |
