Hello there AbsoluementFabuleux,
I was looking into the issue you describe and have an article that may help you get your yahoo account back up and running with the Mail application. It is named OS X Mail: Troubleshooting sending and receiving email messages found here: http://support.apple.com/kb/TS3276.
Open Mail Connection Doctor
In Mail, choose Window > Connection Doctor.
Tip: If you see a spinning gear in Mail or the Mail Connection Doctor, choose Window > Activity to learn what Mail is doing.
Since you will more than likely be getting a red Dot in one place I would recommend following this next section toward the bottom of the article:
Check your Incoming IMAP or POP and Outgoing SMTP email account settings
Use Apple's online Mail Setup Assistant to discover what the correct settings should be for both your Incoming (IMAP or POP) and Outgoing SMTP email servers. If your email service provider is not listed in the Mail Setup Assistant, visit your email service provider's website or contact their support staff to get the correct settings. You can use this "cheat sheet" when asking your email service provider about the settings you need.
After you have the correct email service provider settings, make sure Mail is configured properly.
If all else fails, you can alway remove and re add the Yahoo mail account from the Mail Preferences. This article will help with that, Configuring Mail for your email account found here http://support.apple.com/kb/HT1277.
All the very best,
Thank-you so very very much. I moved around the world and all that that entails just after I posted this (and did get it sorted out, thank-you) so didn't get back to you in a timely fashion and I really apologise. Thank-you so much for looking into this, I really appreciate it. I will also keep this info, as it seems to do this every so often and this is fabulous info.
All the very very best, AbFab