This document explains how to switch between working online/offline for accounts in Apple Mail for OS X 10.9. When an account is taken offline, you will not be able to receive or send messages for that account. However, you will be able to write messages that will be sent from your Outbox once that account is back online. Note: it is recommended that you synchronize/Get Mail for your accounts before taking them offline.
Taking all accounts offline/online
Open Apple Mail, and from the Mail menu at the top of the screen, select Mailbox and click Take All Accounts Offline.
To bring all accounts back online, click on Mailbox from the Mail menu at the top of the screen, and click Take All Accounts Online.
Taking individual accounts offline/online
To take an individual account offline, open Apple Mail, and from the Mail menu at the top of the screen, select Mailbox and click Online Status. Select the account you would like to take offline.
To bring an account back online, select Mailbox from the Mail menu at the top of the screen, click Online Status, and select the offline account you would like to bring back online.