SME v5.6, including the latest updates, all installation defaults are unchanged. Apple Mail.app v1.2.5 on the client. Both the mail server and the client machine are inside a firewall.
When the mail application is first run on the client, the error message in the subject line is often raised. Subsequent attempts to gather the new mail are successful to a degree: Some mail is gathered but many more are retrieved on a third request. After this initial dance, all is well and works as expected.
I find this odd. The OSX firewall is disabled, as are the internet services, so the silly thing should behave. Still, I assume the problem is with the client app. I posted a note to the Apple Unix list, but no one has responded. The silence on the Apple list caused me wonder, so I am posting this question here again.
Any and all ideas, suggestions, and so forth are welcome.