0x800CCC0F is the 'pop3 - sending' error which keeps cropping up. The emails concerned have finally reached their intended destination which is a little baffling. However, when I click the send/receive error message, it tells me that only 3 out of 4 tasks completed successfully.

Do you have a paid Yahoo! account or a free one? I seem to recall that only the PAID accounts can use POP/SMTP.