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Two messages sent using IMAP by two different mail-apps. The appear in the webinterface in the same folder, and visibly everything looks exactly the same. Still, only one is downloaded using Outlook/POP3. Makes no sense to me, except that it could be that the Gmail-app/webinterface adds something to it's messages, which gets it to be handled differently.
GekkeHenkie: thank you for providing detailed answers to the questions.
Ok, I think I understand now the source of the difference that you observe in the quoted portion above.
That difference comes from some specific features (some might call them idiosyncrasies) of Gmail and Gmail POP3:
1. Gmail POP3 downloads All Messages (i.e. including those in Sent),- all as if they were in a single Inbox.
2. Gmail POP3 (by default, but that can be changed) downloads only those messages whose state is "unread" (i.e. not "Read").
3. (If I remember correctly, messages downloaded by a POP3 client are automatically marked read. But this is not important for our discussion here.)
4. [not a standard SMTP feature] All messages sent via Gmail's SMTP are automatically saved on the server in "Sent" folder(label).
5. [rather peculiar feature] Gmail server marks read messages that are sent by a mail client via Gmail's SMTP, but does not mark that way messages sent from the WebInterface.
(Here is the post where I found that:
https://productforums.google.com/forum/#!msg/gmail/LD7xm9IbGT4/zW4yV1mb9N0J )
The rationale for this is that if the mail client sent the message it already has it in "Sent" folder, while the messages sent from the webinterface wouldn't make it to a POP3 client.
And if the client is IMAP3, then it will sync the message in Sent, and the server will actually catch the duplication and eliminate it (so that you don't have doubles).
I
suspect (I haven't read a confirmation to that, but it makes sense), that the same way Gmail does not mark read the messages sent by Gmail app (I am not even sure if they go via Gmail's SMTP or via its proprietary protocol).
So, what we have here is as follows:
a) A message sent from Gmail app: the "Sent" copy is not marked read and for that reason is downloaded by the POP3 client (desktop)
b) A message sent from the POP3 client (obviously, via Gmail SMTP) is marked as read. So, you don't get it back in your POP3 client (so, you rely on the locally saved copy - if you've configured that).
c) A message sent from Aquamail via Gmail SMTP is marked as read. So, it is not downloaded by the POP3 client.
The mystery is solved.
P.S. Note that if all your e-mail clients (mobile and desktop) were using IMAP, you'd have a perfect synchronization of the folders you have on all of them.