An auto-reply is the one piece of automation almost everyone uses and almost nobody configures carefully. That's how reply loops, mailing-list floods and the occasional public announcement that your house is empty for two weeks all happen.
The mechanism is older and stranger than it looks. It isn't a message you send; it's a rule you leave running on a mail server, which then sends messages on your behalf to people you've never seen, while you're not there to notice what it's doing. That's worth thinking about for five minutes before you switch one on.
Here's what a good auto-reply says, what it should never say, and the three settings that stop it becoming a support ticket.
What an Auto-Reply Actually Is
An auto-reply is a server-side rule that watches for incoming mail and sends a pre-written response. It runs at delivery time on the server, not in your mail app, which is the property that makes it useful: your laptop can be shut in a bag in an airport and the replies still go out within seconds of a message arriving.
Underneath it's the vacation extension to Sieve, standardised as RFC 5230. That spec is short, and most of it is about the ways an auto-reply can go wrong — which tells you something about how the feature behaves in the wild.
The same delivery-time execution powers server-side mail filters. An auto-reply is a filter whose action happens to be "send this message back".
The Loop Problem, and How It's Solved
Two people set an auto-reply. One emails the other. Her server replies automatically. His server sees an incoming message and replies automatically. Her server sees that and replies again.
Left unchecked this runs until a queue fills or an administrator notices. It's the classic failure of this feature, and it's why every serious implementation has three defences.
One reply per sender per period. The server remembers who it has already answered and doesn't answer them again for a set number of days. A colleague who emails you six times while you're away gets one auto-reply, not six — which is also just better manners.
Never reply to an automated message. Bulk and system mail carries headers marking it as automated. A rule that respects those headers can't answer another one, which breaks the loop at the first hop.
Never reply to a bounce. Bounces arrive with an empty envelope sender, which by convention means "do not reply to this". Answering one sends mail to nobody and, on a bad day, to whoever the bounce was forged as.
You don't configure any of this. It's worth knowing it exists, because it explains why yours answers fewer messages than you expected — that's the feature working.
Never Answer a Mailing List
The one setting you should always leave enabled is Don't reply to mailing lists and automated messages.
Here's what happens without it. You're subscribed to an industry mailing list with four hundred members. Someone posts. Your rule fires and replies to the list address. Four hundred people receive "I'm out of the office until the 15th". If two other subscribers also have a naive auto-reply running, the list generates traffic on its own for a while.
This is a well-known way to get quietly unsubscribed from a professional community, and it happens several times a year on every large list. The setting costs nothing and prevents all of it.
It works by looking for the headers list software adds — List-Id, Precedence: bulk, Auto-Submitted — and staying silent when it finds them. It also suppresses replies to newsletters, receipts, alerts and notifications, none of which benefit from being told you're away.
Writing One That Helps
A useful message answers three questions the sender actually has: are you gone, until when, and who should they talk to instead.
Good:
I'm away until 15 April with limited access to email. For anything urgent, please contact Sara at sara@example.com. Otherwise I'll reply when I'm back.
Four things make that work. It gives a return date, so the sender knows whether to wait. It names an alternative with an address, so they can act now. It sets expectations honestly — "limited access" rather than a promise to monitor. And it's short enough to read on a phone lock screen.
Now the version people actually write:
Thank you for your email. I am currently out of the office and will respond to your message upon my return. Your patience is appreciated.
No date, no alternative contact, no information. The sender knows exactly what they knew before, except now their inbox has one more message in it.
Some practical notes. Give a date rather than a duration — "until 15 April" beats "for two weeks", because the reader doesn't know when the auto-reply was written. Only name a colleague you've actually asked; discovering you've been volunteered as an emergency contact by an auto-reply is a genuinely annoying way to find out. And if you'll be reading mail occasionally, say so, because a promise of silence you then break confuses people more than either extreme.
What Not to Put In It
An auto-reply answers anyone who writes to you, including people you'd rather not brief on your movements. It's the one message you send with no idea who's reading it.
So leave out the specifics. "Away until the 15th" is fine; "in Portugal until the 15th, back in the office on the 16th" tells a stranger the house is empty and for how long. Social-engineering attacks routinely use out-of-office information — knowing a finance manager is unreachable for a week is exactly the opening a fraudulent payment request needs, and the auto-reply supplied it for free.
Don't publish a colleague's direct mobile number either. Publish a role address like support@ or accounts@ instead of a person, and route it with an alias so the same auto-reply keeps working when the person behind it changes.
And be careful on shared or role mailboxes. One left running on info@ answers every prospect who writes in, which is either exactly what you want or a very public statement that nobody is minding the shop.
Setting It Up
- Open Mailboxes in the dashboard and pick the mailbox.
- Open Settings, then the Auto-Reply tab.
- Tick Enable auto-reply.
- Set a reply subject — "Out of office until 15 April" is better than "Automatic reply", because it's visible in the message list without opening anything.
- Write the message.
- Set a start and end date. Leave them blank and it runs until you turn it off manually, which is how people end up telling clients they're on holiday in November.
- Leave Don't reply to mailing lists and automated messages ticked.
- Save.
The date pickers show your current time zone next to each field. That matters more than it looks if you're setting the dates in advance from somewhere else — the same class of confusion described in calendar time zones, where a wall-clock time with no zone attached means something different to everyone who reads it.
Auto-reply is available from the Starter plan upward. Full walkthrough in the vacation auto-reply guide.
Uses Beyond Being on Holiday
A mailbox nobody staffs. An address that's still on your website but no longer monitored should say so, rather than absorbing messages in silence. An auto-reply pointing at the right address is more honest than a mailbox that swallows enquiries.
Someone who has left. Combine an auto-reply naming their replacement with forwarding to whoever took over. Senders update their records, and nothing is lost while they do. That sequencing matters — set both up before deleting anything, because a deleted mailbox stops receiving mail immediately.
Part-time monitoring. "This inbox is checked on Tuesdays and Thursdays" sets a real expectation and stops the follow-up chasers.
Parental or extended leave. Same shape as a holiday, with a longer date range and a named replacement who has genuinely agreed.
Frequently Asked Questions
Does an auto-reply work when my computer is off?
Yes. It runs on the server at delivery time, so replies go out within seconds regardless of whether any device of yours is on or any app is open.
Will it reply to the same person every time they email me?
No. The server tracks who it has already answered and won't answer them again for a set period, so a colleague who writes six times gets one reply.
Will it reply to newsletters and mailing lists?
Not with the mailing-list setting enabled, which is the default and should stay that way. Answering a list broadcasts your out-of-office message to every subscriber.
Can two auto-replies get stuck in a loop?
Not with the standard protections in place. Automated messages are marked as such in their headers, and a rule that respects those headers can't answer another one.
Can I schedule an auto-reply in advance?
Yes — set start and end dates and it turns itself on and off. Check the time zone shown beside each field if you're setting it from a different region.
Does it send from my address?
Yes, the reply comes from the mailbox it's configured on, so recipients see the address they wrote to.
Can I set one on a shared mailbox?
Yes, and think about it first: an auto-reply on a team address answers everyone who contacts the team, which is a public statement about your availability.
Is auto-reply available on every plan?
It's available from Starter upward. Nano doesn't include it.
