Today, this is the message I get from Thunderbird when I try to send an email.
An error occurred while sending mail. The mail server responded: Transaction failed : Cannot send message due to possible abuse; please visit http://postmaster.yahoo.com/abuse_smtp.html for more information. Please check the message and try again.
I'm on Yahoo IMAP. I just loaded up Mail via IMAP and get no such error message. However, the reason I switched from Mail to Thunderbird in the first place was because
Mail screws up a lot of the email dates, so that a whole bunch of them read 2011-09-09, which is when I switched to Lion on my main machine. Even if I delete the account and reconfigure it, I get the same date on a whole bunch of the messages. So, switching to Mail permanently isn't much of a solution.
If I log into the Yahoo web interface, there is no such error either. This only manifests itself when I try to send from Thunderbird. Furthermore, I have gotten no indication from anyone in my contact lists that they're getting spam from me.