Sudden unexplained change in Gmail and Plusnet mail behaviour

[#1]

My wife and I have identical mail set-ups:  mail arrives at our POP3 Plusnet account which we each manage through Thunderbird on Windows, 2 separate accounts and PCs.

Our mobile devices use Gmail: our Gmail Imap accounts are set to take a copy of mail from Plusnet's server and leave a copy there.  As a way of managing email it might not be to everyones's taste but it works for us, as all our email is in TB on Windows.  So any non-urgent Gmails can be deleted when travelling, say, and dealt with through TB later.

This has worked fine for each of us for years, and still does for me, but for my wife there's suddenly a new unwanted behaviour: if she deletes a Gmail message it is now also deleted from the Plusnet server.  Fortunately I've been able to retrieve them from the Deleted folder in Plusnet webmail and move them back to Inbox where they can then be managed with Thunderbird.  Gmail help suggests deleting the Check Email From Other Accounts setting in Accounts and Import and recreating it: yes, I've ticked the Leave a Copy of Retrieved Message on Server box.  She has just returned from travelling but has had no problems using Gmail on her tablet abroad, is this a clue?  I stayed at home.  I'm completely at a loss.  Thanks

[#2]

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@fm27 wrote:

My wife and I have identical mail set-ups:  mail arrives at our POP3 Plusnet account which we each manage through Thunderbird on Windows, 2 separate accounts and PCs.

Our mobile devices use Gmail: our Gmail Imap accounts are set to take a copy of mail from Plusnet's server and leave a copy there. 

...if she deletes a Gmail message it is now also deleted from the Plusnet server.

I've ticked the Leave a Copy of Retrieved Message on Server box.

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I suspect that potentially there is some fundamental misunderstanding here of the difference between POP3 and IMAP.  A mail box is not POP3 or IMAP, it is simply a mail box.  Email clients connect to the mailbox using either POP3 or IMAP, the behaviour of each is quite different.

From what you've said it sounds as though you have a configuration something like...

        PC                                           Plusnet Mail Box                                         Phone / Tablet

Thunder Bird    <<<< -----------------   POP3 Server

                                                          IMAP Server  << -------------------------------- >>  Gmail

IMAP has no concept of "copying" from the server, nor leaving on the server.  IMAP is a SYNCHRONISATION model where what you do on the client also happens to the mail box contents.  Set up properly what you delete on any device is deleted from the mail server and if so configured moved to the trash folder.

POP3 is a copy or download model.  It has the option of leaving email on the server, but that can lead to complications, especially if the download state becomes confused.  Others a round here have reported getting duplicate of emails on their email clients due to "left on the server" mail items being down loaded repeatedly.

Consequently if you delete over IMAP before the POP3 client has done the down load, you will get the scenario you have described.

If you want to use one mail box from multiple devices, then they all ought to be IMAP configured.  If you then wish to archive emails locally, at a convenient time move them to local folders.

IMAP set up properly also means that you do not need to hunt around all of your devices to find sent mail items.

[#3 · Accepted solution]

Solved! Thank you for taking the trouble to respond.  I believe my understanding of the difference between POP and IMAP was correct but your post made me think more carefully about what I was trying to achieve.  The details below might help someone else who runs into a similar problem.

My wife and I both use Plusnet email accessed by the POP3 server through Thunderbird in Windows.  Since around 2010 we’ve had mobile access through our Gmail accounts.  These are accessed from iOS Mail through the Google IMAP server, which can be configured through web access to copy Plusnet email to Gmail:

Gmail > Settings > Accounts and Import > Check email from other accounts

Using “Edit info” you can instruct Gmail to “Leave a copy of retrieved message on the server”.

This all works well but problems arise sending messages from the correct sender, in this instance picking either the Plusnet or Gmail addresses.  Earlier versions of iOS allowed no direct way to do this but there was a workaround involving pasting a second email address into the settings to construct a comma-separated list.  This is no longer possible.  With no obvious way to create an email address alias as previously, I resorted to setting my wife’s device up with a second account.  This worked, but deletion from the iOS combined Inbox could either cause safe deletion from Gmail – the message would still be available later through POP under Windows – or be disastrous with the message completely gone from the Plusnet mailbox if it had arrived through the Plusnet (IMAP) client on the device.

In fact there IS a way to set up an alias email address in iOS Mail:

Settings > Accounts & Passwords > (the mail account) > Account > Email (in IMAP account information) > Add Another Email

Type the alias address, then press RETURN (essential), then DONE as you climb back up the menu hierarchy.  Note that this will only work if the alias refers to a mail account you have already set up.  So I needed the prior existence of the Plusnet IMAP account on the device before I could create the Plusnet address as an alias, but I could then delete the Plusnet account and the alias would remain.  This is logical from a security perspective.

To have a copy of every sent message: Settings > Mail > (Composing) > Always Bcc Myself

Why not simply have a Plusnet account and drop Gmail?  Two reasons:

1. In the early days using Gmail was a way to achieve Inbox consolidation.
2. Plusnet passwords are not secure. Routing everything through Gmail means that at least the final client stage, on hotel wifi for example, is going through Gmail’s SSL encryption.

And in any case a Google account is useful for other reasons such as syncing contacts and calendars.