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I set Email Accounts A and B to reply with a "Vacation Response" every time a new email is received.

What would happen if Email Account A sent an email to Email Account B?

Would the sending messages back and forth continue indefinitely? Or would something break down? Or would nothing at all happen?

Bonus: On "smart" servers, what could I do with the ability to forward or retrieve mail? (For example, Gmail supports "importing mail" and Forwarding mail in addition to Vacation Responses.)

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Email. Server. Meltdown. Plagues of Frogs... THE END OF THE WORLD... actually, possibly an irate admin, two filled up mailboxes, and a flurry of wasted e mails. – Journeyman Geek May 18 '11 at 23:14
up vote 4 down vote accepted

The answer really depends on how the smtp service is configured. This is a common problem that was solved a long time ago. It does occasionally crop up, but this is usually caused by someone who does not provide email as one of their core services. If both providers planned well then only a couple of messages are exchanged. If neither planned then one or both may be impacted. The impact may range from a simple 'over quota' messages, filled message queues, or complete service melt down.

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Most email services like MS Exchange can be setup and usually are to send only a single reply to an email address per vacation duration. Email A sends Email B a message. Email A receives a vacation email from Email B. Email A sends Email B a vacation note. Since Email B has already responded once to Email A. Nothing happens and the loop is broken.

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Impossible to tell!

This depends entirely on both the way they are set and the servers involved.

I say this because, auto responders can be set on some clients and some servers or third party software (which I guess technically comes under the client banner). It depends entirely on the way they are set - some are dumb and will just go on an endless loop and some are smart and will detect a chain and prevent it.

As for the second part, with the ability to forward pop, you can forward pop! I don't understand what you are asking... If you mean what is forward pop:

If you have a server/port/username/password box:

This is usually an option that some email providers give where you can give the details of another pop server and it will fetch new mail from it and pull it in to your new mailbox.

If it is just the option to turn on/off:

This usually means enable pop support or similar so that you can do the above..

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E-mail servers have functionality to monitor and prevent that from happening. They will only send one reply per message ID. So if the message ID changes, like for a new e-mail, then a second one would be sent.

It is not one per sender per day, etc.

From this Wikipedia article: "Message-ID: Also an automatically generated field; used to prevent multiple delivery"

Can I say that it has never happened, no. But it is designed not to, and works virtually all the time.

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I have observed an auto response loop that went on for about 3 days, with messages every few hours. I'm not sure what interrupted the loop. Most likely action on the remote party's mail admin's part. Our in house solution is only to respond with auto-replies in internal addresses, when the mail server does apparently employ some sort of verification to prevent this repetition. – music2myear Jul 27 '11 at 16:04
@music2myear I only saw it happen once, the last time with Exchange 2003. – KCotreau Jul 27 '11 at 16:37
We've got Domino 6.5 on our server. Which probably explains most issues we have... :) – music2myear Jul 27 '11 at 18:23

Mailservers may use the following headers: X-Auto-Response-Suppress: All (Microsoft Exchange) or Auto-Submitted: auto-generated (RFC 3834) in addition to a unique Message-ID (RFC5322).

However, this alone should not be relied on and mailservers should implement some form of limit to deal with badly behaving mailservers.

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