Cannot Send email - Mail Uses the wrong SMTP server

I have a strange problem. I have 3 email accounts setup. A legacy (yahoo based account) a personal and a business)both gmail accounts). today the business email began issueing a message "cannot send email - useridxxx@gmail was rejected by smtp.YAHOO.com".
As you can guess, I suppose Yahoo would not know about the gmail ID.

I went into mail preferences found the accounts, verified that each account had the correct Incoming and outgoing servers defined to them. I verified the server. I tried to send and received the same message.

I went into Window--> Connection Doctor and everything was green.

Anyone have any idea where this mabe stored (cached maybe - I did restart the iMac after the changes.

Thanks in Advance

Try the tick both under the mail account preferences to "use only this server" under the gmail account