On POP, the messages are downloaded from the server and stored on your computer (also with few exceptions).

When IMAP is used, all e-mails are stored remotely on the server (some exceptions apply). That’s Internet Message Access Protocol (IMAP, for short) and POP (Post Office Protocol). Now highlight any recipients you don’t need anymore. Remove recipients: By default, Mail will remember every e-mail address you enter.Use the Check for New Messages drop-down menu to check for email every hour or so. Indeed, checking your Mail less frequently enables the system to perform faster. Check for the new messages less frequently.Why not to create two mailboxes and split messages accordingly? Sometimes you’ll need to keep hundreds of messages in your Mailbox. To create the new mailbox, choose Mailbox – New Mailbox.

Create a new mailbox (you can call it Archive) and drag messages there from a Mailbox. Archiving them is usually useful if you have messages on a server. Delete all unnecessary messages, don’t keep hundreds of them in your Inbox if you don’t need these letters. Clean out regularly your Inbox and your Sent folder.If you follow these tips, you’ll speed up Mail immensely:

Apple mail download all messages