Problem sending mail to Yahoo Group from Gmail

This problem is not strictly an OSX problem, but I do not know a more suitable site. I belong to a club which uses yahoogroups.com for group mail. Recently every message I send to the group from my MacBook using Thunderbird fails with an error message Your email was not delivered due to a suspected guidelines violation. Please visit http://groups.yahoo.com/local/guidelines.html I might addd that that link fails with 404 page not found which doesn't inspire confidence, but I have no choice but to use Yahoo. Following up other links I found the following:- As a spam prevention measure, email messages sent to Yahoo Groups without SMTP-authentication often result in failed delivery, citing a “suspected guidelines violation.” This happens most frequently because you're attempting to send your message using an external email application (Microsoft Outlook, Mozilla Thunderbird, etc.). You can avoid errors by enabling SMTP-authentication within your email client or email service. I have enabled SMTP-authentication in Thunderbird. I normally use Gmail as my SMTP server and have enabled SMTP-authentication in Thunderbird. I can send mail if I use my ISP as SMTP server, but I can only use this if I am directly connected to the ISP, not when travelling. Can anyone help with this problem?