Mail does not use correct outgoing server

Question

Problem
Mail in Mavericks (and also IOS6 snf 7) does not use the outgoing smtp server associated with the account being used. This results in either a failure to send or mail being put into the wrong "Sent" folder.
Background
I have most of my non-icloud mail pulled down to a home server from which my laptops and ios devices access various accounts using imap. The local home server also provides smtp and relays all mail to my ISP. Recently I have been switching from an old home server (Mac Mini) to a new one (Raspberry pi running the debian Raspbian version with fetchmail,  Dovecot and postfix setup).
While making the changeover I had access to both the old server and the new server enabled on my laptop. Naturally the majority of the settings (Name, email address etc) were the same although the name and address of the server to be used for both incoming and outgoing mail were different. This seemed tp work fine for incoming. However when I tested outgoing mail the copy that should appear in the "sentt" box  went missing. This usually indicates some mistake in the setting of the sent mail folder on the accounts but this all checked out.
I eventually found the missing outgoing mail intended to go through the new server in the sent box on the old server. Further trials showed that no matter what I did the outgoing would always default to the old (and therefore first set up) outgoing server.
Deleting the old account and the old outgoing server of course should cure the problem and will eventually be the solution for me when I kill the old server. But i wanted to get to the bottom of the problem in case it reappeared in other contexts. Checking the "use only this server" box produced failure to send rather than the correct result.
Trialing various alterntaive settings showed that the problem occurs when there are two outgoing servers with the email address in the settings and the user name on the outgoing server name being the same. I had assumed that changing the description field would distinguish between the various settings; however this did not work. Changing the Name Field did work eg by putting (O) after my name.
It appears therefore that mail selects the outgoing server on the basis of email address (with the full name included) and user name (trrespective of the actual server or the Description that shows in the smtp server listing. In some ways this is logical but it produces problems in the context I have described and would also be a difficulty if you wanted to use an alternative server when in a different location.
I have trawled support and elsewhere for any thing similar. Lots of mail problems (don't get me started on the way icloud loses outgoing each time you edit the list!) but I have not found any posts on precisiely this point.
IOS devices seem to have similar problems, but a quick attempt at a similar solution does not work, and I cannot be bothered  to test the options. I will simply clean them out and put in the new accounts.
Advice
Grateful for any comments or advice from people who have encountered similar problems and whether my diagnosis if correct. Have I missed any obvious corrections that would clear this up. I do not know whether this is a Mavericks issue or also appears in earlier OSX versions.
CPE

Answer 1

Peter,
Where ever the Sent and Trash folders show in the Sidebar, highlight first one, and then the other, followed by clicking on Mailbox in the Menubar, and choosing Use This Mailbox For, and choose the function.
Keep us posted on your progress.
Ernie