how to retain emails deleted from the server

[#1]

You illustrate the point well - the user needs to have a plan, implement it and control it.

Using POP3 and IMAP together on the same mailbox needs tech savviness to get something near the desired functionality.  That said, does you solution do what this user wants - have the same email archive history on BOTH devices?

A shared PST file (implies the use of paid for MS Outlook) - if concurrent sharing works - is the only way I can see of achieving the requested functionality without doing separate dual archiving.

[#2]

Thanks @Townman. I'm using MS Outlook for Mac v16.58. I've copied my inbox to a new folder. So if I now delete everything from the server will all the emails in the new folder still be there? 

[#3]

Simple question, but the context is not clear enough!

Where is the "New Folder"?

1. Under a LOCAL PST folder?
2. Under the account for the mailbox?

If (2) and you "delete everything from the server" then no, the files will not still be there, as your "New Folder" is an IMAP folder on the IMAP server.

If the new folder is as (1) then after deleting from the server, the files will still be in the new folder, but only on the device with the local PST file.

I have another strategy for you to consider, which I implemented sometime ago for one of my referrals ... but I had forgotten about.

1. Create a free @outlook.com account (you probably have one already if you have Office365)
2. Configure that as an Exchange account in Outlook
3. Create archive folders in that space
4. Move your Plusnet historic emails to those folders
5. Delete stuff from the Plusnet server
6. ... the email history will then appear on ANY device you configure to use that mailbox

I do similar for a different purpose - sharing contacts, calendars and to do lists across multiple devices.  Thereby any contact or calendar change made across 3 laptops, three tables and my iPhone are all kept in step, more or less instantly without manual intervention.

I do not use that email address for email at all.  On reflection, that might be the most pragmatic way to archive your Plusnet email such that you can still see the whole archive on both machines.  You also do not need to worry about backing up the archive - Microsoft does that for you.

[#4]

I don’t know how Outlook for Mac works but using windows machines I would copy the PST file across and set the deleted option to 3 or more weeks. There’s mileage in @Townman ‘s suggestion of using a remote shared depository.

[#5]

That seems a good strategy @Townman thank you. However I've discovered another problem with IMAP accounts in that unlike POP accounts they are not able to show calendar information. So I'll probably have to do that through an Exchange account as well.

[#6]

POP3 and calendar - some confusion there.  The calendar is not in the email space but in the local email application.  In the case of the ilk of Outlook looking at a Plusnet mailbox that calendar will be just local.

Where the Outlook client is also using an Exchange based email server, the contacts, calendar and to do lists are not held locally, but on the exchange server.  Thereby any other client accessing the same "mailbox" will also see the same contacts, calendar and to do list.

If the device is smart enough to "hook onto" those information resources in its native applications (as can contacts, calendar and reminder on iDevices) the that information will be kept synchronised across all devices.  As I mentioned, I dispensed with the need to duplicate such information on multiple devices a long time ago.

I would note that there are benefits on choosing what you do on which device.  Creating new calendar items I find is done best on the iPhone - it automatically finds similar events (from your history) as you type and entering the location is supported by dynamic address searching, which in turn automatically sets the map location for navigation and travel time alerts.  Such doe snot happen if the entry is created in outlook.

Conversely, tidying up contacts is best done in outlook, especially if you have partial duplicates.

Good luck!