I cannot save sent emails as all my "sent" folders are locked.

Can you explain 'locked' in more detail?

the folder icons in the left column have an envelope with a padlock

The lock indicates a secure connection to the server via TLS. This is what you want.

Thanks.........I need to read more about imap.

You offered NO solution for the fact that his sent folder is not saving sent emails! I have the SAME issue that just started on 8/12/2016. Sent emails are NOT saving a copy to my sent folder. Don't tell me to check to see if I have checked "save a copy"...of course it's checked. Also have verified that I have "subscribed" to all my folders. What now????

Thunderbird has been nothing but one bug after another since I started using it 6 months ago.  :(