Hello SpikesJoubert,
A few Outlook Andoid app versions ago, when I could still add the imap accounts, I had a similar problem.
It was my first reported imap issue to Microsofts support.
When I added the imap account all the folders of the account were visible. But I found that after restarting the app they were not all correctly associated with the app's system folders.
You can tell, when you find certain folders under the header "Folders" instead of above.
imap can have many folders, that you can add yourself like p.e. "MyArchive2020". Some of them have a special function and should be associated with the app's system folders.
p.e. Inbox, Concepts, Sent, Removed items. When they are correctly associated with the system folders, they appear above the "Folders" heading. (The Folders heading might not be there if there are no additional folders to the system folders)
In my case only the Inbox was correctly associated. The Sent and Removed items folders were not.
When you then want to sent an email, the app has no (app) system folder associated to the corresponding imap folder and will not sent. The same for removing an item. If the imap "removed items" folder is not associated with the app's removed items system
folder, it will not remove an item.
The confusing thing is, that under the header "Folder" you will see the imap folders including its content. They just do not function any more as you want them to.
There is more to it. When in previous versions I was able to add an account, directly after addition, there was an "Advanced settings" option right above were you can "remove this account from this device" and "remove this account for all mobile devices"
options. This advanced settings option got to a screen were you could tell what imap folder should used as the send, concepts, removed items unwanted (or spam) system folders. Here you could associate the imap folder with the app's system folder by hand. Once
I used the option, it was no longer available. Also the associations got lost in time and again for me it was no longer possible to sent or remove items. The "Folder"header appears and there under the imap folders that should have been associated with the
system folders
But support asked me to clear the cache again and with the latest version I now can no longer add the account at all.
(Since I have a Dutch version, I had to translate. It can be that you find slightly different names.)
Happy reading