What does ridiculously long message header in an error report mean?
A5040: 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 machine ran out of memory. (Exim stores all the header lines in main memory). 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 HEADER_MAXSIZE in Local/Makefile. Exceeding the limit provokes the “ridiculous” error message. Prior to release 3.022 Exim used two separate limits, one on the length of an individual header line and one on the total number of header lines. A header line longer than 8192 used to provoke the error “Header line is ridiculously overlong”. In subsequent releases there is no limit on individual header lines; only the total matters.
