Apple's Mail application uses icons to delineate virtually every option the software provides. In most cases their meanings are apparent, not only because they use intuitive symbols -- a paper aeroplane for sent messages, a lidless box for in-boxes -- but because they're labelled with text. These labels can, however, be removed, making things less clear. Adding to the problem, some icons are never labelled at all. Knowing what these symbols mean and how to use them will help you better prioritise your e-mails and give you more options when composing messages.
Mac Mail defaults to displaying its tools using icons and labels. If labels do not appear near the icons, it means they've been turned off. You can turn them back on by clicking "View" in the application's top toolbar, and choosing "Customize Toolbar" from the menu options. In the bottom-left corner of the window that appears is a drop-down menu labelled "Show." This menu provides options for either icon and text, text only or icon only. Choose the icon and text option to see the icons with their labels.
Any e-mail in Mac Mail can have a priority assigned to it. These appear in the marker column. This column is to the left of the "From" column of the inbox and is labelled with a flag. There are three levels of priority. A dash means low priority; exclamation marks are normal priority and double exclamation marks are high priority. If a message comes to you with a priority label, it was labelled by the sender. If it does not, you can assign it a priority by going to "Message" in the top toolbar, choosing "Mark" and selecting a priority from the menu provided. To prioritise an e-mail you are sending, click the drop-down menu box labelled with the exclamation mark to the far right of the message window beneath the subject line and choose a priority from the list.
Messages in your inbox may also contain flag or trash bag icons in the marker column. The flag is a generic marker the application provides so users have a way to visually distinguish some e-mails from others based on any criteria they choose. If an e-mail has a trash bag, it means Mac Mail thinks it's junk mail. The application also displays the text in the "From," "Subject," "Date Received" and "Mailbox" lines of these messages in yellow rather than black.
All messages in mail have a status displayed in the column directly to the left of the marker column, marked with a dot. In this column, each message has either a dot, a curved arrow or a blank space. The dot means the message is unread, the curved arrow indicates you've replied to the message and the blank space means the message has been read. Note that Mail automatically marks an open message as read. If you opened an e-mail, but have not read it, you can relabel it as unread by going to "Message," selecting "Mark," and marking it as unread from the options provided.