Sometimes, even if you have your POP e-mail accounts set up in Mail to remove messages from the server immediately (Preferences -> Accounts -> Advanced), there may still be some old e-mail clogging up your mailbox. Glitches in the ether may prevent all messages from getting deleted; an unstable connection to a mail server, for example, could cause this, as could a problem with Mail. While a few messages won’t cause problems, if there are a lot—and if some contain large attachments—they could eventually fill up your mailbox.
Mail has an Account Info window that lets you view what is actually on your mail server, rather than what is in Mail itself. To access this window, Control-click anywhere in the mailbox list and select Get Info. If you have more than one e-mail account, a popup menu at the top will let you choose from your accounts. If there are any messages in your mailbox, whether new or old, they’ll show up in the list section of the window.
You can select individual messages, or select multiple messages in the standard way. (Shift-click to select a group, and Command-click to select non-contiguous messages.) To delete the selected messages, click the Remove from Server button.
Another way this is useful is if you’re traveling and only have dial-up access. If you want to sift through your messages and delete spam before downloading your e-mail, you can do so from this window. This will save you time, and let you read just the valid e-mail.