I'm brand new to Thunderbird. I have set up a Yahoo Mail account in Thunderbird and have no problem receiving e-mails via this account. However, when trying to send a message from this account, if, under Account Settings, I insert a Reply-to Address I get this error message: "Sending of the message failed. An error occurred while sending mail. The mail server responded: Request failed; Mailbox unavailable. Please check the message and try again." If I delete the Reply-to address either in Account Settings OR in the e-mail I am trying to send, the message goes ok. This does not happen with another, non Yahoo, account I have added to Thunderbird. I'm lost as to why this might be and any helpful ideas as to how I can overcome the problem would be very welcome.