SMTP problem

I hope someone is out there, still thinking about Mulberry...

I am a long-term user, but with limited technical expertise. I use Mulberry 
as my primary email (IMAP) client on my 3 PC's. On 2 of the 3, it works 
without a hitch.

But, on the third, I have a persistent problem with the SMTP account, even 
though the setup is identical to the other 2 PCs. One feature of my setup 
on all 3 PCs is that, on the SMTP account, I use the option to "hold 
messages when added to the queue" so I can double-check the message before 
it is really sent.

I am afraid I cannot give a simple description of the problem, but here are 
a couple of symptoms:

-if I push enable/disable on the SMTP account, I get a "timed out" while 
connecting to server error.

-if I decide to edit a message in the queue, and then hit "send," I get the 
error message "File in use elsewhere while appending message to a mailbox" 
for the SMTP account.

If I shut down Mulberry and restart, it works OK for the first Email, but 
then the problem immediately recurs. I have tried uninstalling and 
reinstalling, but to no avail.

I would be grateful for any suggestions. Many thanks!
Tim Walsh