I'm just getting around to reading all the helpful messages regarding my problem and will be trying out each of the suggestions. A very big thanks to everyone for the supportive help. I'll let you know what I discover.
In the meantime I wanted to mention one additional thing. I am switching mail over from a Solaris server to a newer Linux box and Outlook on this Windows 7 machine has and continues to successfully communicate with the Solaris system, downloading all the mail. I can take the mail file from the Linux /var/mail directory, transfer it over to the Solaris machine, point the Windows machine at that server, and it downloads the mail without a problem, so if Outlook is part of the trouble, it is due to some unique interaction with dovecot. All file permissions and ownership are identical on both servers. So there is some subtle issue here that I just haven't nailed down.
Regards,
Jeff