Fix Manual IMAP Copy Dates in Thunderbird

Thunderbird makes it deceptively easy to migrate emails between IMAP accounts. Add both accounts, select messages, right-click, "Copy To." Done. Except the dates are now wrong on the destination server - and Thunderbird does not warn you about this.

When Thunderbird uploads a message to the destination IMAP server, it issues an APPEND command without specifying the original INTERNALDATE. The destination server does what any IMAP server does in that situation: it records the upload timestamp as the INTERNALDATE and adds its own Received header dated right now. The original Date header inside the email body remains intact, but the server-level metadata is permanently overwritten.

Here is the deceptive part. Thunderbird's default column layout shows the "Date" column, which pulls from the Date header - the one that is still correct. So right after copying 8,000 messages, you glance at Thunderbird and everything looks fine. The dates appear normal. Problem solved, right?

Wrong. The server-side INTERNALDATE is corrupted. And that INTERNALDATE is what every other email client uses. Connect Outlook to the same account? Migration date on every email. Open Apple Mail? Same problem. Even within Thunderbird itself, adding the "Received" column reveals the true damage. Server-side IMAP search commands (SEARCH SINCE, SEARCH BEFORE) reference the INTERNALDATE, not the Date header. Your email history is silently broken at the protocol level, even if Thunderbird's default view hides it from you.

In Thunderbird's default configuration, the visible damage is minimal - the "Date" column still looks correct. But enable the "Received" column and the corruption becomes obvious: every copied message shows the copy date. Server-side sorting by received date produces nonsensical chronological order. Thunderbird's Quick Filter bar, when filtering by date ranges, queries the server using INTERNALDATE and returns inaccurate results for copied messages.

The real danger extends beyond Thunderbird. Any other client connecting to the same IMAP account (Outlook, Apple Mail, mobile email apps) will display the wrong dates by default. Backup tools that archive via IMAP capture the corrupted INTERNALDATE. Message filters in Thunderbird that trigger on received date use the INTERNALDATE, potentially misfiling messages. Redate.io's header chain analysis and date metadata reconstruction process corrects the INTERNALDATE on the server itself, which means every client that connects afterward - Thunderbird, Outlook, mobile apps, backup tools - sees the correct dates without any additional configuration.