Mail and multiple accounts in Gmail

Solution 1:

  1. Open accounts tab in preferences
  2. Open any mail account you have
  3. Drop down "Outgoing Mail Server (SMTP)"
  4. Select existing account or select "Edit SMTP Server List" to create and/or modify some.

Solution 2:

In the mail account settings inside Mail, you can specify several mail addresses for an account (separated by commas). When you then send mail from this account you can select the mail address to use in the message window.