How are messages entering the other folders? I presume they come in to inbox and are then moved? Capturing all incoming and outgoing is doable, but I don't understand the concern for other folders and how they are updated. Please explain.

Hello @davidsk . Many thanks for your help. It's really not unusual. Picture how any standard webmail account (yahoo, gmail, etc.) works:

• Messages are moved from the inbox to other folders.
• Additionally, sent mail is stored in the SENT folder.

Does this answer your question ?

That's my point. Capture the inbox prior to moving to other folders. A simple filter can copy all incoming to local folder and you could set sent folder to local folders . This is doable. If you focus on the message at the time of arrival, it's easy; if you insist on capturing online folders after messages have been moved to them, you abandon the concept of capturing all messages.

@davidsk : I am very thankful for your help, but sorry I don't understand what you say. The goal is to capture and save all messages ever going in and out of an email address of a webmail account. I was hoping that my original posting precisely laid out all requirements. But apparently it is unclear? Can you please ask me what is unclear? If this can be done in Thunderbird, could you outline how ?

I think I did. You are not defining your complete requirements. My dear wife uses POP and captures all incoming and all outgoing. . Folders other than inbox are subordinate to inbox and only have input when moved from inbox.

You are not defining your complete requirements.

What is incomplete? We do not even have a solution for the current requirements (let alone something I may have omitted unintentionally).

My dear wife uses POP and captures all incoming and all outgoing.

I don't see how this is possible, given that POP3 does not understand the concept of folders at all. (Hence why POP4 was developed).

Folders other than inbox are subordinate to inbox

This is an interesting special case, but as far as I understand, even in this special case, POP3 only sees the Inbox, not any other folders or subfolders, because POP3 has no notion of folders. But even if POP3 were useful in your special case, it doesn't help me, because I have no control over the folder structure in the webmail account. For example, the sent mail is stored in the SENT folder, which is NOT a subfolder of the Inbox.

You have yet to describe how the non-inbox folders are being propagated. If from the inbox, then my solution works.

@davidsk : Why does it matter how the folders are populated with messages ?

A standard folder structure could look like this, for example:

• INBOX: for incoming messages
• SENT: messages sent via the web interface are placed here
• SPAM: spam folder (mail server automatically moves messages there)
• bills: custom folder (populated via filters or messages are moved here manually from the INBOX)
• todo: custom folder (populated via filters or messages are moved here manually from the INBOX)

But I really need a solution for any arbitrary folder structure.

• IMAP mirrors any arbitrary folder structure, but suffers from the problems I mentioned in the OP: 2-way sync (mirror) whereas I need 1-way sync (backup).
• POP3 provides 1-way data transfer, but does not know any folders except the INBOX.

Therefore I don't know how to backup the messages from all folders.

So, you want to capture everything, regardless of whether read or sent or deleted by thunderbird, such as phone or website. Your only choice is IMAP. Set the account to keep everything, including trash and spam. Then, periodically, download everything to your PC, as the IMAP server allocation may hit its max. Then, delete all messages no longer needed online. The second time you do this, also do a scan for duplicates as you merge the new download with the prior one. This gives you a process to administer your desire to do this. OR, you could invest in server-type software that does daily backups of entire account and set up your own email server.

@davidsk: many thanks for your help!

So, you want to capture everything,

yes

regardless of whether read or sent or deleted by thunderbird, such as phone or website.

I am enquiring if there is a way to use Thunderbird for continuous, incremental backup. As such, Thunderbird must not delete any messages from the webmail account (as already stressed previously).

Your only choice is IMAP.

That's what I thought (and feared). The problem is that IMAP does 2-way sync (mirror) whereas I need 1-way sync (backup)! This is stated in the OP.

Then, delete all messages no longer needed online.

Since IMAP does 2-way sync, this would delete the messages from the webmail account, which – again – must not occur!

The second time you do this, also do a scan for duplicates as you merge the new download with the prior one.

Have you omitted a step in your instructions? Because nothing in your instructions so far would have created duplicates. Duplicates are only created if I COPY messages to another folder. But you did not say anything about copying yet ...

OR, you could invest in server-type software that does daily backups of entire account

I could use regular backup software to back up Thunderbird's IMAP account, sure. But that doesn't solve multiple problems laid out in the OP:

1. I would have to do backups every couple of minutes to have a chance to capture messages before they are deleted via webmail. This is not feasible.
2. I cannot locally delete any messages (because this would delete the messages from the webmail account as well, with which I MUST NOT interfere with). So I will have many GB of data sitting around in my Thunderbird profile and I am unable to delete any messages there.