Gmail – Bookmark Gmail conversation

Apparently, this is still an issue. Definitely feels like a bug.

Interestingly some email clients do something similar ...

Yes, if Thunderbird recognises the reply as a follow-up, it ignores the Reply-to (or, at least, it did the last time I tested it).

... however I agree that if the Reply-To header is there, it should be honored in all cases

bkennelly claims that, based on the assumption you intended to follow up to a message you sent, the software "preserves, rather than reversing, the address headers." However, this isn't actually what Gmail does in the issue discussed here. Remember that this Gmail behavior occurs whenever the "From:" header in the received message matches any of your "Send mail as" addresses, although it isn't necessarily the address that the message was sent "To" or the default address for your account. Then, when you click "Reply," Gmail preserves the "To" of the original message but not the "From" of the original message. This shows further that it's a bug -- not a deliberate attempt to preserve headers of the original message.

If you can import the EML files into a mail client that support IMAP this will be easy.

It seems that Thunderbird can do this, but I'm not 100% sure. (Don't have a local copy to play with.)

Anyway, set-up Thunderbird to be an IMAP client for Gmail. (Lifehacker has an excellent guide to do this.)

Then, import your EML files into Thunderbird.

If necessary, move them to a temporary folder (called "Zimbra" perhaps) which will be a label in Gmail. Then just wait for IMAP to sync them all to the cloud.

After that you can delete Thunderbird, keep it around to make local archives of your e-mail, or keep using it as your main client.

I was able to do this a couple of years ago with thousands of messages from my Pegasus Mail client. (There was some pain in converting to a format that Thunderbird could read, but getting the messages into Gmail was the easy part.)