There are add-ons that aim to select an appropriate identity to use as the "From:" address when replying. Thunderbird tends to prefer the primary identity associated with the account currently open, and add-ons can modify this behaviour by seeking an identity within the selected message and then using the appropriate identity when composing a reply. If you were moving messages to Local Folders, which I think is preferable to moving them into a real email account's Inbox, then it would offer the default identity. I think there are also bugs in Thunderbird which make automatic reply address selection erratic; Reply All in particular seems to make a total mess of choosing a relevant From: address. e.g. https://addons.mozilla.org/en-US/thunderbird/addon/correct-identity/?src=api However, your problem illustrates a major disadvantage in the business of moving messages into a communal Inbox; messages' provenance tends to be lost, and if this is important to you, then it may be better to leave messages in their original Inbox and use some other way to manage them. I use Saved Searches to collate newly-arrived or unread or project-specific messages, so their actual location become less significant. A common reason for doing as you have done is a fear that new messages will be missed, or are hard to find, being scattered over multiple accounts. A built-in feature intended to address this is the '''View|Folders→Unified''' option, which collates common folders into virtual folders. I don't use it myself, and can't recommend it, as it has, in my opinion, several bugs, including inaccurate message counts, which make it unreliable. https://support.mozilla.org/en-US/kb/using-saved-searches

Thank you, this solved my problem.