How do I fix 553 error?

The message could not be sent because the server rejected the sender's e-mail address. The sender's e-mail address was 'Hardcastle@TexasEmbassy.com'. Subject 'Re: Grandfriends Day', Account: 'TEC', Server: 'smtp.att.yahoo.com', Protocol: SMTP, Server Response: '553 From:... show more The message could not be sent because the server rejected the sender's e-mail address. The sender's e-mail address was 'Hardcastle@TexasEmbassy.com'. Subject 'Re: Grandfriends Day', Account: 'TEC', Server: 'smtp.att.yahoo.com', Protocol: SMTP, Server Response: '553 From: address not verified; see http://help.yahoo.com/l/us/yahoo/mail/original/manage/sendfrom-07.html', Port: 465, Secure(SSL): Yes, Server Error: 553, Error Number: 0x800CCC78

I have 2 computers set up exactly the same way--one works fine, this one gives me 553 error. Only started in last 2 weeks or so.

There are 2 email accounts on OE 6, only Hardcastle@TexasEmbassy.com is troublesome.

This configuration has been in place for years--last change were those required by Yahoo regarding SSL. I have been through the processes in the url.
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