Thanks for your help.

I'd already done as you suggested - my settings are exactly as you describe. I've got another issue, and I wonder if the two are somehow related.

In my main account, email isn't received unless I've selected the InBox (I've got dozens of folders and use rules to sort my incoming mail). If I'm reading mail in any of the other folders, I have to click on my InBox to receive more, at which time lots of mail is often received.

Any ideas?