re :I have the Thunderbird client set up for IMAP. Whenever I delete an email from that client, the email disappears from the Spectrum server

You are using IMAP, so you see a remote view of the server folders. Basically, the server folders and the ones in Thunderbird are one and the same. They are synchronised. If you delete off server via webmail it will get deleted off Thunderbird imap mail account. If you delete in Thunderbird using an imap mail account it will delete off server. This is not a Thunderbird quirk, it is how all imap connections work.

Emails are stored on server, imap account in Thunderbird lets you remotely see what is on server, read, delete etc and as the folders are synchronised, the server and imap mail account will be updated to show the same view.

re : I can delete from my mobile client (IMAP) and it remains on the server. Then it is not imap, it is pop.

I'm starting to understand, but the mobile client is set up as imap (at least it says it is). Charter insists on imap, but how do people use various devices to access the account without affecting each other (delete from one device but have it show up in ... my wife's device?)

If different people on different computers are accessing the same account / same email address using an imap connection, then any changes that any person makes are made on the server, so everyone can see the changes.

That is the purpose of imap mail accounts. You need to work with the system by using/creating additional folders and agreeing a system on how to use them which everyone understands.

Senario: Person A opens and reads email, it is something they are responsible for dealing with, so they move email into a suitably named folder and process it. Perhaps that folder is a folder in that persons name showing who has opened and is dealing with that email. Everyone can see that Person A is working on that email. It does not stop others from reading it.

Person A opens and reads email and decides it is not for that person to deal with, so they mark it as 'unread' and leave it in Inbox. So others can see it is still marked as unread.

Person B wants a copy of everything, own personal record kept separate from server record. They right click email in imap folder and select to put a copy in 'Local Folders' folder. Or perhaps they synchronise entire imap folder for offline use, then in offline mode copy a batch of emails into 'Local Folders' account. The copied email in 'Local Folders' is no longer in an imap folder, the original is still in imap folder marked as unread. Opening the copy does get marked as read, but it will not effect any imap folder. So server still shows as unread.

Toad Hall, thank you so very much for taking the time to spell things out so clearly. (Google search had given me articles that were misleading at best) Now that I understand, and have some good suggestions I'll work out a solution to our situation. Kent

It might be worth mentioning that the email clients on phones don't seem to implement "pure" IMAP. A phone has limited resources and you wouldn't want its precious memory to be filled with email. OTOH, your PC has multi GB of disk space so can afford to be quite generous with the amount of storage it can allocate to email. Having a local copy in an email client means you can browse messages faster and search them, so there are clear advantages to having local copies.

A phone has to find a compromise; it needs to store some number of recent messages, but not going back for all eternity, and maybe not all the subfolders containing messages. It can fetch them on demand, but won't store them for long. And recognizing these space limitations, it may delete local copies of messages but won't pass that delete request back to the server, as the email client on your PC would.

<tl;dr> The email clients on phones don't work like those on "proper" computers.