Server Fault is a question and answer site for system and network administrators. Join them; it only takes a minute:

Sign up
Here's how it works:
  1. Anybody can ask a question
  2. Anybody can answer
  3. The best answers are voted up and rise to the top

I have some questions about custom mail headers

Does anybody know of any mailserver which deletes proprietary/custom headers when being transmitted to its destination?

Is there a header you can set in order to force a mail client to return a certain header when sending a reply to a message that has exactly this header?

Does a custom header always begin with X- ? Is it safe to use others?

Is it generally discouraged to use custom headers? How else could I transmit information that doesn't belong to the body and shouldn't directly be seen by the user?

PS: I know there's a similar question but this doesn't fully response to this question.

Thanks and regards

share|improve this question
up vote 5 down vote accepted
share|improve this answer
Also duplicate headers can be added. This is relatively common if email passes through the same anti-spam software on multiple servers, or if the original messages had forged anti-spam headers. By convention custom headers always begin X- ad this prevents name collisions with standard headers. – BillThor Mar 4 '11 at 21:12
The most common example of duplicate (even triplicate or more) headers, is the Received: header. – adamo Mar 4 '11 at 21:16

I've never seen anyone use anything other than X to proceed a custom header. I can't say with absolute certainty that other letters won't work but I certainly wouldn't count on it.

Microsoft Exchange 2010 includes a feature called a header firewall that strips X-headers from untrusted sources (the internet) and this sort of thing exists in some SMTP security gateways too. Many admins will strip almost all non-critical headers off a message as it leaves to obscure any internal information.

X- headers are very commonly used by SMTP security software. Usually they add a few headers that explain the message's rating by the antispam/antivirus engines that scanned it. But, these things are done most of the time as a message is received inbound to a mail system. I haven't seen them used much on messages that you send out to the internet.

I get the sense that you're hoping to push the use of SMTP headers beyond where they were ever intended to go.

share|improve this answer
In terms of Internet mail, the Thread-Index: header that is used by Exchange and Outlook for mail threading is a custom header that does not begin with X- – adamo Mar 4 '11 at 21:17
As is Thread-Topic. – james.garriss Feb 21 '13 at 17:04

Your Answer


By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.