A custom header that doesn't exist elsewhere in the wild will be distinctive to you and hence a strong signal of not being spam.
If you can name a feature of email, spammers have used it to try to deceive filters; this exact logic would suggest to a spammer "hey, I can use an exotic header to make myself look like a real person!". I wouldn't be surprised if any sort of unrecognized (or at least uncommon) header would be grounds for a very high spam score; I'd love to be proven wrong about this though (I've never tested it).
Not sure we're on the same page regarding conversations. Where Thunderbird shows you single messages, Gmail-in-a-browser shows you "conversations", which are really single threads including both your messages and other peoples'. When viewed through IMAP, however, it doesn't appear that way at all; you see each individual message as a line item like any other email server, and that leaves Thunderbird to apply its own threading if appropriate.
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Date: 2012-08-02 12:53 am (UTC)If you can name a feature of email, spammers have used it to try to deceive filters; this exact logic would suggest to a spammer "hey, I can use an exotic header to make myself look like a real person!". I wouldn't be surprised if any sort of unrecognized (or at least uncommon) header would be grounds for a very high spam score; I'd love to be proven wrong about this though (I've never tested it).
Not sure we're on the same page regarding conversations. Where Thunderbird shows you single messages, Gmail-in-a-browser shows you "conversations", which are really single threads including both your messages and other peoples'. When viewed through IMAP, however, it doesn't appear that way at all; you see each individual message as a line item like any other email server, and that leaves Thunderbird to apply its own threading if appropriate.