Thunderbird silently blanks emails after IMAP-to-Exchange switch

Since switching to Thunderbird's new Microsoft Exchange support announced on November 18, 2025, 24 messages moved from Microsoft's server to local storage have ended up completely blank, much like the macOS Mail bug extensively covered by Michael Tsai. All 24 show a file size of 127 bytes, containing only:

X-Mozilla-Status2: 00000000
X-Mozilla-Keys:

Granted, this is with Maildir enabled and hundreds of thousands of messages across dozens of folders, but the bug had never appeared when the account was configured as IMAP in Thunderbird, so have switched back for now. (The new account had to be set up manually, then switched to OAuth for both IMAP and SMTP, as it wasn't offered otherwise (setup in a fresh Thunderbird profile worked normally). One downside of returning to IMAP: syncing changes to macOS Mail once again requires quitting Thunderbird.)

Update

While penning this post, happened upon an open bug from 17 years ago:

Thunderbird "loses"/corrupts email messages when downloading from the mail server to a local folder

❧ 2026-04-02