Type: string
Default: "%i; from %a on %{!%a, %b %d, %Y at %I:%M:%S%p %Z}"
This specifies the format of the In-Reply-To header field added when replying to a message. For a ful llisting of defined escape sequences, see the section on index_format.
Note: Don't use any sequences in this format string which may include 8-bit characters. Using such escape sequences may lead to bad headers.