Set Mail preferences
Use Mail Preferences to set or change options for the email accounts you use in Mail, and for working with messages and mailboxes. In Mail, choose Mail > Preferences, then click a preference pane to view options:
General: Change settings that range from how often Mail should check for messages to the kinds of messages to include when you search in Mail.
Accounts: Add, delete, and disable your email accounts in Mail. Use the Mailbox Behaviors pane to specify where to store draft, sent, junk, deleted, and archived messages. Use the Server Settings pane to set options for the incoming and outgoing mail servers.
Junk Mail: Enable the junk mail filter and change settings to improve it.
Fonts & Colors: Select the fonts and font sizes used to display messages.
Viewing: Change settings to control how messages are shown in the Message viewer.
Composing: Change settings for composing, addressing, and responding to messages.
Signatures: Create signatures that can be automatically appended to the messages you send.
Rules: Create rules to do things like have messages from a certain person automatically moved to a mailbox, or highlight messages about a birthday party.
To learn about the options in a pane, click the Help button 
at the bottom of the pane.