Thanks Barney, but selecting the correct email from the inbox list was not my problem. I had been careful to select the account from my inbox list and select the correct account from the "From"drop down within the message.
So I called Apple support and they fixed the issue.
So if anyone is still having this problem, try this:
Click on "Mail" and open "Preferences"
Under the Accounts menu, select the first email account from the list in the column on the left.
Under the "Account Information" tab for that email account, click on the pull down menu for "Outgoing Mail Server (SMTP): and choose "edit SMTP server list...
Double click on the text under the "Description" column, and give each email account a unique name ie. work, personal etc. Click OK and save changes when prompted.
This fixed my problem.
If, like me, you want all your outgoing mail to come from the same address,
go to Mail > Prefs > Accounts
Select the account you want all mail to come from, (on the left)
Click on the pull down menu for "Outgoing Mail Server (SMTP):
Choose your preferred account
Then click the "Use only this server" check box.
Job done