Custom headers in Email lost?

• Mail servers can be programmed to insert or remove any header along the way
• 822 promised that the IETF would never define field names beginning with X-. It did not prohibit use of non-X names by other organizations.
• Please try not to use custom headers that do not begin with X-

Update: Deprecating the "X-" Prefix and Similar Constructs in Application Protocols

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.