I HAVE FOUND OUT WHAT CAUSES THIS ISSUE AND HAVE A SIMPLE FIX.
I am not a regular member of this forum, but after spending so much time browsing and testing, I felt obliged to give you guys the answers you seek.
1) ROOT FOLDER
Each IMAP server uses a folder structure to store your mail ....
a "root folder" which is the top level path name
individual folders for sent mail, incoming mail, etc
For example, the root folder name for Plesk systems is "Inbox". For Gmail it is "[Gmail]" - all case sensitive. (that's Inbox and [Gmail] - ignore the parentheses).
If you tell your email client the "root folder path", it will correctly identify all of the folders that exist on the IMAP server. This is an item in Settings, and I have found it in Outlook, Outlook Express, and on my Android phone. So if you have a Plesk IMAP email account, tell your Outlook Express, your Thunderbird, and your Android mail client that the root folder name is "Inbox", and it will correctly display the IMAP folder structure (below example for Plesk) ....
Inbox
sent-mail
Drafts
2) ANDROID BUG
Now, here is the tricky part. When you send an email from your Android phone, using an IMAP account, it ALWAYS puts the sent mail into a folder called "Sent", regardless of where your IMAP server normally puts them. The good news is that if you have done Step 1) above correctly, a new folder called "Sent" will show up on the IMAP server, and then in all of your email clients. So now you have ....
Inbox
sent-mail
Drafts
Sent
In the above example, you end up with most sent emails residing in "sent-mail", but those sent from your Android in "Sent".
Email clients such as Outlook are clever enough to put sent mail into the correct IMAP folder, eg. "sent-mail" for Plesk. But your Android is not clever enough, and it will put them into "Sent", thereby creating a new "Sent" folder on the IMAP server.
If you have correctly entered the root folder path, the above folder structure, complete with the new "Sent" folder, will correctly appear in your email client. Emails will be correctly sync'd, so that emails sent from your Android appear in all of your mail clients, albeit in the "Sent" folder on their own.
If you can follow the instructions in this thread, you may be able to override the default "Sent" folder name, but I have not done so. Now that I know what is going on, it is not much of an issue to have emails sent from my phone in a separate folder: they are visible on Webmail, on my PC, and any other device, provided it is properly configured ; and you can always move them if you wish.
Hope this is of use!