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145 lines
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145 lines
6.4 KiB
Markdown
[#]: collector: (lujun9972)
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[#]: translator: (wyxplus)
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[#]: reviewer: ( )
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[#]: publisher: ( )
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[#]: url: ( )
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[#]: subject: (How to detect automatically generated emails)
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[#]: via: (https://arp242.net/weblog/autoreply.html)
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[#]: author: (Martin Tournoij https://arp242.net/)
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How to detect automatically generated emails
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======
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### How to detect automatically generated emails
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When you send out an auto-reply from an email system you want to take care to not send replies to automatically generated emails. At best, you will get a useless delivery failure. At words, you will get an infinite email loop and a world of chaos.
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Turns out that reliably detecting automatically generated emails is not always easy. Here are my observations based on writing a detector for this and scanning about 100,000 emails with it (extensive personal archive and company archive).
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### Auto-submitted header
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Defined in [RFC 3834][1].
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This is the ‘official’ standard way to indicate your message is an auto-reply. You should **not** send a reply if `Auto-Submitted` is present and has a value other than `no`.
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### X-Auto-Response-Suppress header
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Defined [by Microsoft][2]
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This header is used by Microsoft Exchange, Outlook, and perhaps some other products. Many newsletters and such also set this. You should **not** send a reply if `X-Auto-Response-Suppress` contains `DR` (“Suppress delivery reports”), `AutoReply` (“Suppress auto-reply messages other than OOF notifications”), or `All`.
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### List-Id and List-Unsubscribe headers
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Defined in [RFC 2919][3]
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You usually don’t want to send auto-replies to mailing lists or news letters. Pretty much all mail lists and most newsletters set at least one of these headers. You should **not** send a reply if either of these headers is present. The value is unimportant.
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### Feedback-ID header
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Defined [by Google][4].
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Gmail uses this header to identify mail newsletters, and uses it to generate statistics/reports for owners of those newsletters. You should **not** send a reply if this headers is present; the value is unimportant.
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### Non-standard ways
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The above methods are well-defined and clear (even though some are non-standard). Unfortunately some email systems do not use any of them :-( Here are some additional measures.
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#### Precedence header
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Not really defined anywhere, mentioned in [RFC 2076][5] where its use is discouraged (but this header is commonly encountered).
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Note that checking for the existence of this field is not recommended, as some ails use `normal` and some other (obscure) values (this is not very common though).
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My recommendation is to **not** send a reply if the value case-insensitively matches `bulk`, `auto_reply`, or `list`.
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#### Other obscure headers
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A collection of other (somewhat obscure) headers I’ve encountered. I would recommend **not** sending an auto-reply if one of these is set. Most mails also set one of the above headers, but some don’t (but it’s not very common).
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* `X-MSFBL`; can’t really find a definition (Microsoft header?), but I only have auto-generated mails with this header.
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* `X-Loop`; not really defined anywhere, and somewhat rare, but sometimes it’s set. It’s most often set to the address that should not get emails, but `X-Loop: yes` is also encountered.
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* `X-Autoreply`; fairly rare, and always seems to have a value of `yes`.
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#### Email address
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Check if the `From` or `Reply-To` headers contains `noreply`, `no-reply`, or `no_reply` (regex: `^no.?reply@`).
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#### HTML only
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If an email only has a HTML part, but no text part it’s a good indication this is an auto-generated mail or newsletter. Pretty much all mail clients also set a text part.
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#### Delivery failures
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Many delivery failure messages don’t really indicate that they’re failures. Some ways to check this:
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* `From` contains `mailer-daemon` or `Mail Delivery Subsystem`
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Many mail libraries leave some sort of footprint, and most regular mail clients override this with their own data. Checking for this seems to work fairly well.
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* `X-Mailer: Microsoft CDO for Windows 2000` – Set by some MS software; I can only find it on autogenerated mails. Yes, it’s still used in 2015.
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* `Message-ID` header contains `.JavaMail.` – I’ve found a few (5 on 50k) regular messages with this, but not many; the vast majority (thousends) of messages are news-letters, order confirmations, etc.
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* `^X-Mailer` starts with `PHP`. This should catch both `X-Mailer: PHP/5.5.0` and `X-Mailer: PHPmailer blah blah`. The same as `JavaMail` applies.
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* `X-Library` presence; only [Indy][6] seems to set this.
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* `X-Mailer` starts with `wdcollect`. Set by some Plesk mails.
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* `X-Mailer` starts with `MIME-tools`.
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### Final precaution: limit the number of replies
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Even when following all of the above advice, you may still encounter an email program that will slip through. This can very dangerous, as email systems that simply `IF email THEN send_email` have the potential to cause infinite email loops.
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For this reason, I recommend keeping track of which emails you’ve sent an autoreply to and rate limiting this to at most n emails in n minutes. This will break the back-and-forth chain.
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We use one email per five minutes, but something less strict will probably also work well.
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### What you need to set on your auto-response
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The specifics for this will vary depending on what sort of mails you’re sending. This is what we use for auto-reply mails:
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```
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Auto-Submitted: auto-replied
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X-Auto-Response-Suppress: All
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Precedence: auto_reply
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```
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### Feedback
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You can mail me at [martin@arp242.net][7] or [create a GitHub issue][8] for feedback, questions, etc.
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--------------------------------------------------------------------------------
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via: https://arp242.net/weblog/autoreply.html
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作者：[Martin Tournoij][a]
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选题：[lujun9972][b]
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译者：[译者ID](https://github.com/译者ID)
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校对：[校对者ID](https://github.com/校对者ID)
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本文由 [LCTT](https://github.com/LCTT/TranslateProject) 原创编译，[Linux中国](https://linux.cn/) 荣誉推出
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[a]: https://arp242.net/
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[b]: https://github.com/lujun9972
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[1]: http://tools.ietf.org/html/rfc3834
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[2]: https://msdn.microsoft.com/en-us/library/ee219609(v=EXCHG.80).aspx
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[3]: https://tools.ietf.org/html/rfc2919)
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[4]: https://support.google.com/mail/answer/6254652?hl=en
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[5]: http://www.faqs.org/rfcs/rfc2076.html
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[6]: http://www.indyproject.org/index.en.aspx
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[7]: mailto:martin@arp242.net
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[8]: https://github.com/Carpetsmoker/arp242.net/issues/new
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