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.
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).
### Auto-submitted header
Defined in [RFC 3834][1].
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`.
### X-Auto-Response-Suppress header
Defined [by Microsoft][2]
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`.
### List-Id and List-Unsubscribe headers
Defined in [RFC 2919][3]
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.
### Feedback-ID header
Defined [by Google][4].
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.
### Non-standard ways
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.
#### Precedence header
Not really defined anywhere, mentioned in [RFC 2076][5] where its use is discouraged (but this header is commonly encountered).
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).
My recommendation is to **not** send a reply if the value case-insensitively matches `bulk`, `auto_reply`, or `list`.
#### Other obscure headers
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).
*`X-MSFBL`; can’t really find a definition (Microsoft header?), but I only have auto-generated mails with this header.
*`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.
*`X-Autoreply`; fairly rare, and always seems to have a value of `yes`.
#### Email address
Check if the `From` or `Reply-To` headers contains `noreply`, `no-reply`, or `no_reply` (regex: `^no.?reply@`).
#### HTML only
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.
#### Delivery failures
Many delivery failure messages don’t really indicate that they’re failures. Some ways to check this:
*`From` contains `mailer-daemon` or `Mail Delivery Subsystem`
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.
*`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.
*`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.
*`^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.
*`X-Library` presence; only [Indy][6] seems to set this.
*`X-Mailer` starts with `wdcollect`. Set by some Plesk mails.
*`X-Mailer` starts with `MIME-tools`.
### Final precaution: limit the number of replies
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.
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.
We use one email per five minutes, but something less strict will probably also work well.
### What you need to set on your auto-response
The specifics for this will vary depending on what sort of mails you’re sending. This is what we use for auto-reply mails:
```
Auto-Submitted: auto-replied
X-Auto-Response-Suppress: All
Precedence: auto_reply
```
### Feedback
You can mail me at [martin@arp242.net][7] or [create a GitHub issue][8] for feedback, questions, etc.
