Suddenly I am having issues sending emails using the Thunderbird client:
1st explanation: you need to change your password somebody tried to hack your account
2nd explanation: somebody on your computer did something dodgy.... the server has blocked you.
3rd explanation: everything is fine it is your email client use outlook
3th explanation: everything is fine use yahoo mail
4th explanation: you receiving and sending too much spam (received 13 spam emails in 4 months!!!! - clean computer no malicious activity)
5th explanation 4 hours later - Yahoo are having an issue with their servers with thunderbird: but everything is fine just use webmail (I would if my broadband connection would be stable enough!!!!!!!!!!!)
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Its a problem with Yahoo. It seems that someone in their spam filter team decided that anyone with a reply-to field must be a spammer. Admittedly domestic users shouldn't need to use a different reply-to address, but they should have done a simple test that a reply-to address that is identical to the sender address is OK.
So you need to remove the "reply-to" field from Thunderbird
Click on "Tools"
Click on "Account Settings"
Click on the name of your account
Delete the content of the "reply-to" field. It will revert to greyed out text.
If you have mulptiple identifties on one account, then you need to click on "Manage Identities", then for each identity click edit and delete the "reply-to" for each identity.