Take email accounts online or offline in Mail on Mac
An email account can be offline (not connected to the Internet) if your Internet connection is disrupted, a mail server experiences problems, or you purposely take it offline (for example, prior to air travel).
In the Mail app on your Mac, do one of the following:
Take accounts online: Choose Mailbox > Take All Accounts Online or Mailbox > Online Status > Take [specific account] Online.
Take accounts offline: Choose Mailbox > Take All Accounts Offline or Mailbox > Online Status > Take [specific account] Offline.
When your account is offline you can’t get or send messages, but you can write messages to send later. You can save your message as a draft and send it when your account’s online again; or click Send to let Mail store your message in the Outbox and automatically send it when your account’s online.