The error message has nothing to do with OpenSSL itself, so you need to stop focusing on that. The error is coming from a failed SMTP MAIL FROM command, long after an SSL/TLS session has been negotiated, and the Yahoo account has been logged in to. Though, I do find it interesting that the 2nd line of the exception's Message (which is supposed to contain the actual SMTP error message from the server) is basically nonsense, it doesn't look like a valid SMTP error. Which could indicate a corrupted SMTP communication.
Something else I notice - in the source code of xmailer's TSendMail.Send() method, when it calls Synapse's TSmtpSend.MailFrom() method, it is setting the Size parameter to an incorrect value. It is setting the parameter to the character length of the Sender's email address, but it should be setting the parameter to the byte size of the email message instead. So, that could potentially be causing Yahoo to reject the command.