Webmail folder structure - problem with sent items
FIXED
I may well have created this problem myself, in trying desperately to get sent emails to appear in mail clients on two Windows 10 PCs and a Windows 10 phone.
I don't any longer have a top-level Sent folder alongside Drafts, Spam and Trash. I say "any longer" because I'm sure there used to be one. I don't really see how I could have deleted it or moved it myself but am willing to take the blame! The screenshots show the structure in Webmail.
When I send using Webmail, all is fine - messages are saved in the webmail Sent folder and synch to Sent in Outlook 2016 (PCs) and Outlook Mail (phone); when I send using Outlook 2016 or Outlook Mail, my emails are received but they are not saved anywhere unless I opt to save copies in the local Sent Items folder in Outlook 2016; there's not that option on the phone app, so they just disappear.
Is this anything to do with the mail clients looking unsuccessfully for a top-level Sent folder to sync to? If so, can that be recreated?
No, but restoring things as they should be requires some tricky manipulations. What follows is a suggestion for doing this, but not having needed to do it I've no idea if following the steps which follow has undesirable side effects. That is a risk you will have to accept.
First thing to note. The main folder is called INBOX (in capital letters) which by convention (RFC 3501) is recognised as the storage folder (mailbox) where incoming messages will go. In the Plusnet implementation all other folders are subfolders of INBOX. This means their folder names are INBOX.folder1, INBOX.folder2, etc, with the full stop used as folder separator.
Webmail designates certain folders as special, these being INBOX.Drafts, INBOX.Sent, INBOX.Spam and INBOX.Trash. The links in Special Folders settings assign purposes to these which reflect their names. You've managed to insert another folder level to your Sent folder so it is currently called INBOX.INBOX.Sent. Note that the second INBOX there is a subfolder name, it has no connection with INBOX at the first level. Restoring normality requires that second level name to be removed.
To do this, log into webmail, click the cog icon bottom left and select Manage folders, This should give the folder display shown in your first image. Because the folder is currently designated as special you can't simply rename it, it needs to be changed to non-special first. I suggest creating a new folder as a place holder for this by clicking the + sign, then typing (just) testfolder in the Folder name dialog; Make sure Parent folder shows as --- then click Save
This should return you to the folder list with testfolder at the bottom (you may have additional folders).
Now change to the Preferences tab and select Special Folders. Use the selector against Sent to set testfolder for this role and click Save
Return to the Folders tab. You should find INBOX.testfolder is now shown as a special folder with the postbox icon and I expect INBOX.Sent will no longer be special (just Sent shows in my case in the image below)
Now select INBOX.Sent to display its properties. Change the Folder name to just Sent with Parent folder ---; then click Save.
That should return you to the folder list with the Sent folder listed simply as Sent (as in my image above). Choose the Preferences tab and Special Folders. Set the Sent selection back to the (renamed) Sent folder and click Save.
This should return INBOX.Sent to its previous role with the postbox icon. testfolder becomes an ordinary folder again. You can delete this by selecting it in the folder list, clicking the cog icon and choosing Delete.
Hopefully if you now return to the webmail mail list things will be back to normal. You will, of course, have to make sure other email clients work properly, changing their settings if necessary.
Unfortunately the Windows 10 Mail app plays by different storage rules from most, if not all, other email clients and more importantly the folder structure Plusnet (and probably many other ISPs) provide. The only common feature is that INBOX (in capital letters) is recognised as the storage folder for incoming messages. My suggestion is to use a different email client.
My webmail is to all intents and purposes untouched simply because I do not use it but it doesn't have the structure detailed above.
@Paul_Beck Were problems with folder access permissions discussed in a previous topic of yours ever resolved?
@Browni Thanks for illustrating the default state which others are likely to find. 
That display shows the special folder names assigned by webmail, not the real folder names. The association is set in the Special Folders settings page accessed by clicking Settings then Special Folders. The names on the left are webmail names, the names in the selection boxes are real folder names. Above the list is a checkbox for the "Show real names for special folder" option. With default settings that checkbox is not set and users see webmail names for the special folders. What was shown earlier in this thread is what users see if the checkbox is selected.
Though setting the checkbox shows special folder names as INBOX.Drafts, INBOX.Sent, etc, any additional folders created by the user display without the INBOX. prefix. That doesn't mean it isn't there, for example INBOX.testfolder is the real name of the folder I created in my previous post, as illustrated when I temporarily made it a special folder.
@Browni wrote:
I'm not asking a question @spraxyt, I'm pointing out my untouched settings are different.
Hey ho.
@Browni Apologies, I appreciate that and could have phrased my reply better. 
I've revised my text to reflect that the post aims to explain differences users might see when using webmail with their own settings.
@Paul_Beck I'm pleased to hear your message manipulation problems were resolved. However the inability to create new folders you experienced earlier suggests your access permissions on INBOX might not be correct. That might prevent you renaming the INBOX.INBOX.Sent folder to INBOX.Sent.
Do you have any other non-special folders? I realise that should be unlikely if you don't have folder creation permission on INBOX. 
@Paul_Beck I wonder if that parent Inbox is at the second level? Webmail appears to get confused if one has a second level INBOX. I created one to experiment, then created another level. When I made that special (in place of Sent) the folder properties show as follows:
The Folders pane display is like that for Sent in your first picture. Do the Folder properties for your Sent folder show its name as INBOX.INBOX.Sent?