(In reply to Kent James (:rkent) from comment #130)
> Any other functionality is very difficult to implement given the current design of the backend, (snip)

Phenomenon itself is pretty simple;
- If filtering upon fetch of new mail download, custom headers are fetched
  because used custom headers are placed in mailnews.customHeaders by
  message filter definition, and Tb's IMAP code includes the custom headers in
  uid xx fech BODY[HEADER.FIELDS (DATE FROM ... custom_headers ...)],
  then filter can process the custom headers.
- If after the fact filtering, and if custom header is not held in offline-store
  file, Tb doesn't request re-fetch of custom header and try to use locally held
  data(.msf, offline-store file), then filter can't obtain  custom header data.

So, any of following can be a solution, although some of them have issue of "Repair Folder is needed".
(note:)
(Offline-Use=On  here == auto-sync is enabled  && selected for offline use    )
(Offline-Use=Off here == auto-sync is disabled || not selected for offline use)
a) Force saving custom header data in .msf by setting in mailnews.customDBHeaders
   in addition to current mailnews.customHeaders upon filter definition.
b) If custom header is used, save custom header data in offline-store file upon
   fetch even when Offlne-use=Off folder, in conjunction with change to "fetch
   body[HEADERS]" from "fetch body[HEADER.FIELDS]", and use the header data in
   offlie-store file even when Offline-use=Off folder.
c) If custom header is used by filter, and if the custom header is not held in
   offline-store file, and if it's Work Online mode,
   issue uid xx fech BODY[HEADER.FIELDS (custom_headers)] according to
   mailnews.customHeaders
d) If custom header is requested, and if IMAP Offline-Use=Off folder, and if
   filter execution request is not "when Checking new mail" only,
   ask user to change the folder to Offline-use=On or not,
   and if answer is no, reject the rule with message of considerate apology.
   In addition to it, even when Offline-Use=On folder, if retention setting for
   offline-store file size limitation is used, ask user,
      Can you promise that you will never complaint about bug 184490?
   and, if answer is no, don't accept the rule.  
e) When custom header is fetched upon initial fetch by mailnews.customHeaders,
   if all fetched headers is saved in Disk/Memory cache when Offline-use=Off,
   and if Disk/Memory cache is accessed when custom header is missing,
   problem of this bug may be resolved, but I'm not sure.

I perefer d) if short-term or mid-term solution, and I think d) is kinder to user than current bypass of "hiding Body at filter definition panel" and d) is applcable to "Body filter upon IMAP fetch" case too(see bug 806308 for Body only and bug 199689 for customized header and Body, please).

From perspective of implementation workload only, a) is simplest and easiest. But I don't think it's safe, because it's easily increases memory requirement and may cause performace problem. "setting in mailnews.customDBHeaders", which is known as a simpler workaround of bug 402594 than "filter rule of custom header of Received", should be done by user as an available workaround of problem like this bug with sufficient understanding about impact.

From perspective of preferable solution, I think b)-way is best. And, b) is helpful for implementating solution of "IMAP Body filter" case.

Kent James, any of above is very very difficult to implement in near future?