Disable Any Auto-Responder
To turn off any auto-responding rule you have set up in Mail and stop automatic replies from going out:
To have Mail reply only to emails you received at a specific address, set the criterion to To contains me@example.com. To auto-respond only to senders in your Contacts, to people you have emailed before or VIPs, make the criterion read Sender is in my contacts, Sender is in my previous recipients or Sender is VIP respectively. To have the auto-reply sent to all incoming emails, make the criterion Every Message.
For a vacation or out-of-office auto-reply, tell the recipient when you expect to return. If you don’t plan to go through your old mail when you return, let recipients know when to re-send their messages if they’re still relevant. If you click Apply, Mail will send the auto-reply to existing messages, generating possibly thousands of messages and multiple identical responses to the same recipient.