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When I delete email from my Gmail account, it first goes to the trash folder.
When I then empty that, where does it go?
And how does the technology work behind the scenes?

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Excellent question. I wonder if Google really ever deletes anything, they probably just hide it from your screen. Data mining and user profiling is what pays for this service after all. When gmail first came out, there was not even a delete button at all. – Thilo Oct 19 at 5:21
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Only slightly related study by the University of Cambridge, where they deleted pictures from photo sharing sites to see how long they remained accessible (mostly due to how CDN work): lightbluetouchpaper.org/2009/05/20/… – Thilo Oct 19 at 5:46
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They go to heaven. – alex Oct 19 at 7:22
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@alex: to be precise, Silicon Heaven en.wikipedia.org/wiki/Silicon_Heaven – Andrew Grimm Nov 10 at 23:30

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8 Answers

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Gmail Privacy Notice (current September 12, 2008, but also present in the October 14, 2005 version which is the first I can find) clearly states "Residual copies of deleted messages [..] may remain in our offline backup systems":

Your choices

  • [..]

  • You may organize or delete your messages through your Gmail account or terminate your account through the Google Account section of Gmail settings. Such deletions or terminations will take immediate effect in your account view. Residual copies of deleted messages and accounts may take up to 60 days to be deleted from our active servers and may remain in our offline backup systems.

So, what does may imply here...

Despite this promise to delete emails after at most 60 days, in 2006 a US court did order Google to provide emails from their backups. According to CNET's Police blotter: Judge orders Gmail disclosure:

What: In a lawsuit brought by the Federal Trade Commission, a subpoena is sent to Google for the complete contents of a Gmail account, including deleted e-mail messages. [..]

[..]

Outcome: Judge grants subpoena and orders that all e-mail messages, including deleted ones, be divulged.

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may probably implies 'It would be technically infeasible to overwrite at the platter level all deleted data the instant it is deleted, so we're allowing ourselves a reasonable amount of leeway to take care of such things in a way that doesn't grind the system to a halt' :) – bdonlan Oct 19 at 7:52
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Straight to the FBI.

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+1 Absolutely the correct answer. :) – Robert Harvey Oct 19 at 4:47
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The fact you're deleting emails, is a give-away you have something to hide ;) – Nick Josevski Oct 19 at 5:20
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Or you get a very large quantity of large email attachments, which has taken you close to your (very large, yet finite) quota. Or you end up with a lot of clutter that is not spam, but you simply want to delete anyway to keep "All Mail" a little more readable. – samjetski Oct 19 at 10:07
This is the best theory. – Cawas Mar 11 at 19:25
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Google doesn't really make a habit of telling everyone how their services work internally. But if we are to assume gmail is built upon BigTable, which is likely, then most likely when you delete an email, the actual record is not purged immediately. Gmail simply marks in BigTable that the email is gone, and later, in a periodic compaction routine, the tablet is rebuilt without that email in it.

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That is if deleting an email deletes the record (as opposed to just setting some flag). But as you say, the people who know are not going to tell... – Thilo Oct 19 at 5:40
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Into the void. I doubt gmail is any different from any other email system. As others have mentioned there may well be backups and the messages may not be purged the moment you hit the delete button. But this would be for Google's benefit in case of a failure rather than a means for an end user to recover lost data. On the other hand if you are worried about the security of your messages then I am not sure using a service like gmail would be a particular wise decision in the first place.

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They are removed forever. So you can't retrieve them.

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Sure about that? See NSD's answer. – Robert Harvey Oct 19 at 4:48
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The FBI/NSA/CIA/ (add your paranoia inducing acronym here) can retrieve the mail, but the OP cannot. – lexu Oct 19 at 5:08
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Even if there are no deals with government institutions, and even if Google has no interest in keeping the emails for their own purposes, it is still more work to really delete them, then to not delete them, so I am not so sure that they are "removed forever". Even if they do get flushed out eventually, that probably does not happen in a timely fashion. – Thilo Oct 19 at 5:26
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Good mail goes to Heaven, where it dances in the clouds with no care in the world. It will for eternity be transported where ever it wishes to go without SMTP error and there will always be eyes ready and willing to read it.

Which level of hell the bad mail goes to depends on its type, as described by Dante, much of it finding its way to the 8th circle.

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Might be funny, but such kind of answer should remain in comments, not as an actual answer ;-) – Gnoupi Oct 19 at 9:29
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It goes straight to the FBI indeed, but also:

Wallmart, MacDonalds, Coca Cola, Sony, and any other paying company! ;)

Google does not really delete anything younger than 7 years. After that time, even the offline backups may contain parts of the information. However, if you send out an email unencrypted, many mailservers might hold copies of that in their transaction logs.

EDIT: Google does not keep it for 7 years. But in Holland/Europe, ISP's must keep data records for 7 years. It is a Brussels thing as we europeans call it... That said, the other large comapnies have their ways to gather these kinds of information of the record... :)

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Any sources? If Google claims it's deleting after at most 60 days, then where's the 7 years coming from? – Arjan van Bentem Oct 19 at 8:16
I don't know where "7 years" comes from, but Google did previously not guarantee deleting anything after any particular length of time. As GMail has only been publicly active since 2004 it won't get have any data 7 years old, though other Google services may do. IIRC the 60 day thing is a relatively recent addition to the relevant policies, partly in response to concerns publicly discussed by privacy advocates. – David Spillett Oct 19 at 8:48
I updated my answer with a reference to an older version of the Gmail Privacy Notice (the same promise in 2005; I couldn't quickly find an earlier version). And also a link to a court ruling telling Google to get data from the backups... – Arjan van Bentem Oct 19 at 9:23
Google does not keep it for 7 years. But in Holland and I think also in Europe, ISP's must keep data records for 7 years. It is a Brussels thing as we europeans call it... That said, the other large comapnies have their ways to gather these kinds of information of the record... :) – Aaaaaaaaaha ERLEBNIS Oct 20 at 8:14
In The Netherlands tax regulations require a 7 year period. That's the excuse to keep the data from the public transport cards for 7 years, just because some people apply for tax reduction. Yes, no jokes, 7 years! However, ISPs need to keep their (limited) records for 1 year, and soon for just 6 months, which is the European minimum. Even more: the are obliged to permanently remove the data soon after that period expires. (And all this is paid for by the ISP, thus their clients.) Telecom operators also need to keep the records 1 year, but might in fact keep billing information a bit longer. – Arjan van Bentem Oct 20 at 8:46
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I work at google and although I am not in the gmail team I have worked with them (see video chat) previously and I can assure you that deleted mail go to a very happy place.

Seriously now, deleted emails are at first just hidden from you and physically deleted only when the actual disk space they occupy is needed.

However there is an inside joke of the undeleted: Emails from Page and Brin cannot be deleted. Ever

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So how does that fit into Gmail's statement that Residual copies of deleted messages and accounts may take up to 60 days to be deleted from our active servers? Anyone understands that the bits on disk may not be cleared, but given this statement I'd want Gmail to delete the pointers to those bits within 60 days. – Arjan van Bentem Oct 23 at 15:26

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