On 4.10.2010, at 19.53, Tom Hendrikx wrote:
It seems Mail.app uses the timestamp ('arrived date'?) on the mail file itself (Maildir here) to display the time.
When your MUA accesses the mail over IMAP, it does not see the Maildir file's mtime. Your MUA should use the Date header in the e-mail, or the date/time when it first sees the message (f.i. when not Date header is available).
IMAP protocol has a concept of "INTERNALDATE", which contains the time when the message was received. With Maildir it's the same as the file's mtime.
With Mail.app you can configure it to show/sort messages by "Date Received" (=INTERNALDATE) or "Date Sent" (=Date: header). I prefer the "Date Received" so that a bad Date: header won't put the mail in the middle of the mailbox.