Thunderbirdでの手動IMAPコピー日付の修正

Thunderbird is a popular free メールクライアント often used for manual email migration between IMAP accounts. Users add both the source and destination IMAP accounts to Thunderbird and copy or move messages between them. When Thunderbird uploads a message to the destination IMAP server via APPEND, it does not pass the original INTERNALDATE parameter. The destination server sets the INTERNALDATE to the upload timestamp, and adds its own Receivedヘッダー with the current date.

Thunderbird's date display behavior depends on the column configuration. By default, Thunderbird shows the "Date" column (from the Dateヘッダー) rather than a "Received" column. This can partially mask the problem: if a ユーザー only sees the Date column, the dates may appear correct because the Dateヘッダー is preserved during the copy. ただし、 the underlying INTERNALDATE on the server is wrong, which affects server-side sorting, search, and any other client that connects to the same account.

The issue becomes fully visible when the ユーザー adds the "Received" column to Thunderbird's message list, or when they access the same メールボックス from a different client (Outlook, Apple Mail) that displays the Received date by default. The corrupted INTERNALDATE on the server persists に関係なく which client is used to view メールボックス, and it affects all IMAP operations that reference the message date.

In Thunderbird, the default "Date" column shows the email's Dateヘッダー, which is preserved during the copy and may display correctly. ただし、 Thunderbird's "Received" column (if enabled) shows the IMAP INTERNALDATE, which displays the copy date. Server-side sorting in Thunderbird (which sorts by INTERNALDATE when sorting by received date) produces incorrect chronological order for copied messages.

Thunderbird's message search can produce mixed results depending on whether it searches locally cached data or performs a server-side IMAP search. Local searches may use the Dateヘッダー (correct), while server-side searches use the INTERNALDATE (incorrect). Thunderbird's message filters that trigger on received date use the INTERNALDATE, potentially misrouting or misfiling copied messages. Quick Filter bar date filtering also uses server-side dates, returning inaccurate results for copied messages mixed with normally-received ones.