Keep IMAP messages locally when deleted remotely

Question

I use my email from my phone and my computer via IMAP. I want to set something up so that if I delete a message via my phone, my computer will still keep the message locally.

For example, assume I leave my computer on, with a synchronize interval of 5 minutes. I want to be able to send something from my phone, wait 5 minutes to be sure my computer has downloaded the item from the Sent folder, then delete it from the IMAP sent folder via my phone, but have the computer at home keep it's copy.

Is this possible with any readily available email clients out there? I have Thunderbird and Outlook at the moment, but would be willing to learn a new interface for this feature. How can I accomplish this?

In response to RedGrittyBrick's comment:

The purpose is to maintain a complete, automatic (offline) backup of every message to pass through the account. For various reasons, certain messages may not remain on the IMAP server, but do need to be kept in offline storage.

Answer 1

The danger of what you are trying to achieve is that if the connection from the computer to the IMAP server fails, the sent message may be lost. Also, although IMAP is supposed to work fine when accessing from different machines at the same time, some implementations don't behave well.

This said, a possible solution with Thunderbird is as follows: Set the email account to use its own folder (not Local Folders). Then set a designated folder, say saved-sent-from-imap in Local Folders. This is where the sent messages will be kept. Note that this folder is not on the IMAP server. Make sure that the IMAP Sent (whatever the actual name is) folder has "when getting new messages for this account, always check this folder" checked. Then set a filter to move all messages from the IMAP Sent folder to saved-sent-from-imap.

Edit: this way you won't have to delete the sent message from the phone; so you can't accidentally delete a message that is not moved to the local folder.

Answer 2

As a general rule, no. The whole point of IMAP is that the server contains an authoritative copy of the mail and that if clients store any messages at all they're simply caching them for other reasons (e.g. indexing or performance).

If you want to archive your mail, your best is to find something dedicated to that mission. Tools like MailStore are designed for this.

You could probably abuse Thunderbird, Outlook, or many other clients into copying all new IMAP messages to local folders using their built-in rules/scripting/filter systems.

You could run your an IMAP server on your own PC (such as hMailServer) and then use a tool to just copy everything to it without deleting missing messages from the destination. (imapsync may do this, compiled Windows versions available here; BaGoMa or other tools may also be pressed into service for this)

Answer 3

I suspect a similar effect could more easily be achieved by

1. always bcc yourself
2. configure the phone NOT to add sent stuff to the sent folder