Turn on a vacation response so anyone who emails you gets an automatic reply letting them know you’re away and when you’ll be back — and set it to switch itself on and off so you don’t have to remember.
Turn on a vacation response
Open Vacation response
Click the Settings gear. In the left margin under Set up, click Vacation response.
Enable it
Turn on Enable vacation response.
Write your message
In Respond with this message, type the note people should receive while you’re away.
Set the dates
Set From and Until dates so it starts and stops on its own (or leave Now / Forever).
That’s it — no Save button
Your reply is active for as long as the toggle is on.
There’s no “Save” button — and Fastmail reminds you it’s on. Settings take effect as you make them. While it’s active, Fastmail shows a banner at the top of your mail with a shortcut to switch it off, so you won’t forget.
How it behaves. Fastmail replies to the same person no more than once every 3 days, only replies to mail with your address in To or Cc, and never to spam, mailing lists, or forwarded mail.
Two gotchas. The vacation reply needs Spam Protection turned on to work (it is by default — leave it on). And Fastmail won’t auto-reply to messages from your own account, so test it by sending from a different address.