Zimbra has an internal recieved date based on IMAP INTERNALDATE. We use this for sorting. Most mail clients also use this. Thunderbird is in the minority and ignores INTERNALDATE then uses the Date header in a message. This is a bit more expensive since even to sort an inbox you'd need to fetch and parse each message to get the Date header. As for PST files you should be using the PST import tool and not trying to drag messages in via IMAP. This tool was designed to handle the conversion of PST and it's better suited for the job.
I can see a possible enhancement may be to have an import mode or something so when you drag messages in from a client like Thunderbird and no INTERNALDATE is set populate it with the Date header. Via zmlmtinject this can be set by the x-zimbra-received header. The server will then use that header's value as the internal date. |