This addon adds a new column called X-Original-To in the message list which shows the original recipient address of the message (the contents of the X-Original-To header). This can be useful for systems with catch-all email addresses.
Install from the Mozilla add-ons page, or use build.sh to produce an unsigned XPI file to install manually.
The new column can be selected as soon as the addon has been installed, and all new messages will automatically have their X-Original-To header recorded.
However, there will be no information for existing messages unless the database is rebuilt for the desired folders (Folder Properties > Repair Folder). Note that rebuilding a folder will reset its column layout and sort order.
Note that this repo uses git submodules, so it needs to be cloned using the --recurse-submodules option. If this is skipped, then git submodule update --init can be used after the fact.
This addon uses the HeaderColumns, LegacyPrefs, and ex_runtime experiment APIs.