jader
Here is one approach.
Generally speaking you need to copy messages in batches for each folder, one folder at a time (using the Linux command prompt) and then fix ownership.
Due to the folder structure created by the mail system ie each folder name has system subfolders of cur, new, tmp, and the folders need to be named in the format .../Maildir/foldername.subfoldername, it's an easier approach to create the main folder structure in a mail client. By doing that you will automatically create those necessary system subfolders with the correct name format & system subfolders.
Then at the command prompt copy the message files in each folder to the corresponding folder in the userB .../Maildir/....
eg
For userB in a mail client, create the main folder named userA-mail
Then (still using the mail client for userB) under that folder create all the original subfolders ie Inbox, Archives, Sales, Technical etc.
Then use the command prompt to copy the messages one folder at a time ie
cp /home/e-smith/files/users/userA/Maildir/cur/* /home/e-smith/files/users/userB/Maildir/.userA-mail.Inbox/cur
cp /home/e-smith/files/users/userA/Maildir/.Sales/cur/* /home/e-smith/files/users/userB/Maildir/.userA-mail.Sales/cur
cp /home/e-smith/files/users/userA/Maildir/.Sales/new/* /home/e-smith/files/users/userB/Maildir/.userA-mail.Sales/new
and so on, if a folder ie any .../new or .../tmp does not have any messages in it, then no need to copy that
Then fix ownership (not permissions) by doing
cd /home/e-smith/files/users/userB/
chown -R userB:userB Maildir
Do not overlook the . (dot) at the beginning of the folder names, and that each folder/subfolder is named in the format .userA-mail.Sales
You could use midnight commander (mc) & copy the messages from mc window to mc window to make this process a little easier.
The other approach is where you have copied the userA Maildir folder in totality to userB.
I think this is what you have done and is the source of your problem (ie only "seeing" the Inbox).
I think the problem with this method is that the subfolders are not named correctly, and in the wrong location.
As mentioned above, subfolder names are like .../Maildir/.userA-mail.Sales/
When copying the folders create the new folders in the format as above, which really means you have to copy & rename each folder & subfolder, either manually or with a script.
I recall the dovecot index files are automatically recreated with correct indexing etc.
Edited with corrections.