What does ridiculously long message header in an error report mean?

There has to be some limit to the length of a message's header lines, because otherwise a malefactor could open an SMTP channel to your host, start a message, and then just send characters continuously until your host runs out of memory. (Exim stores all the header lines in main memory while processing a message). For this reason a limit is imposed on the total amount of memory that can be used for header lines. The default is 1MB, but this can be changed by setting <small>HEADER_MAXSIZE</small> in Local/Makefile before building Exim. Exceeding the limit provokes the ridiculous > error message.

---