I have a similar problem, but under Windows 10. If I use outgoing port 465 to send a message, I get the first error message attached below. If I use port 587 instead, I get the second error message instead. Note that the first error message says that there is a problem with the outgoing server setup, but with no hint of what this problem is. Creating and using a digital key made no change in how the incoming server operates - it still downloads messages properly. I found and deleted the saved passwords for both the incoming server and the outgoing server for this account; the next attempt to download messages asked for the password, and I entered the digital key as instructed. This direction now works properly. Attempts to send messages always fail, without asking for a new password. The third image is from Yahoo's instructions on account setup. I got the first two lines of the outgoing server setup correct, but cannot find any TB information on whether I have the last two lines set correctly or not.