In which case would an email client alter the message data of local archives?

An email client will alter the message data of local archives primarily when converting messages between formats. This conversion process typically involves changing the encoding or structure of the email data, which can include transforming the email from one format to another, such as from MIME to plain text or changing email clients' proprietary formats. Such alterations may affect how the email is stored and displayed in the client's local storage.

In contrast, sending large attachments does not inherently modify the message content itself; the client may handle these attachments differently, such as breaking them into smaller parts, but the core message data remains intact. Similarly, during synchronization with the server, the email client generally updates local archives to match the server's current state, but this does not involve changing the actual message data. Lastly, deleting unwanted messages is a straightforward action of removing data from local storage rather than altering it.