have you checked in the subscriptions that there is a folder called sent on the server? (Right click the account in the folder list and select subscribe)

Most issues with sent folders, and deleted folders, is they are not mapped correctly. See Tools menu (alt+T) > account settings > copies and folders for Send and Tools menu (alt+T) > account settings >Server settings for Deleted.

Thunderbird assumes things like Trash and send are the folder names on the server. But the server my use "deleted" and "sent mail" and therefore trouble arises. Correctly mapping them to what we see as blindingly obvious associations may be all you need to do.

Thanks for the suggestion. All of this seems to be fine. I actually got everything to work on a Linux machine but not on my Windows 8.1 laptop, so I am wondering if there is an issue with Windows 8.1.

Please clarify a couple of points. 1. Are you getting an error message when you send? If so, what kind? 2. In your Copies/Folder settings - in the first option where it says 'when sending messages automatically', is the box next to 'Place a copy in' ticked? And after that, which option is ticked and what are the details? (**Note: Don't provide personal info, for your personal email, simply put myemail@... if it is correct.)

1. I am not getting any error message. A window pops up saying "Copying message to Sent" that never goes away.

2. The "Place copy in" is ticked, and it says "Sent on" my e-mail server.

3. Oddly enough, after 3 days of this problem, the problem went away just a few minutes ago. Therefore, it looks like I might be all set.

That's great! I scared your computer into behaving! I'm soooo awesome. :) :) If the problem comes back, be specific about the settings you have and what you have checked. There are some really great tekkies on this forum (no, not me) and the more info you provide the more accurate their advice.