Mail (Yosemite): If a mailbox is offline
To show the Mail sidebar, click Mailboxes in the Favorites bar.
Take a specific account online: Click the lightning bolt icon , or choose Mailbox > Online Status > Take [account] Online.
Take all accounts online: Choose Mailbox > Take All Accounts Online.
If you can’t take an account online, make sure you’re connected to the Internet. If a warning icon appears next to the account name, click it to open Connection Doctor. Learn more about using Connection Doctor.
Some mailbox names can appear dimmed if you don’t have permission to access them, such as public mailboxes on a mail server.