Sent Items for IMAP go to Storage Folder instead of Server

This issue has been discussed previously, but never answered correctly.  Apparently the technicians who respond always seem to assume that it relates to Gmail which is NOT the case for me.  The issue is that I had a POP email account for many years and then set up an IMAP account with the same provider.  My old messages were preserved in a local Storage Folder.  It works fine, except that whenever I send a message, the copy of the message keeps going to "Sent Items" in my Storage Folder, not the "Sent Items" in the IMAP folders.  I have checked the option to use the special online IMAP folders and done every other setting manipulation I could try, but I cannot get the sent messages to go into the IMAP "Sent Items" folder rather than the local Storage Folder.  Any suggestions? (Again, I am not using Gmail, etc., I am using Cox.)  Thank you.