Yosemite Mail will sometimes show Gmail outgoing mail as being offline when you first setup a Gmail account. To fix this open up Mail and do the following steps:
- Open preferences and go to Accounts.
- Select “Edit SMTP Server List” from the “Outgoing Mail Server Dropdown”.
- Select “Advanced” and put your account password in the password input.
- Voila, all fixed.
For some reason, Mail doesn’t always save the password here on setup, so these are steps required to fix it.