I had a problem with my Mail.app for last few days. All over sudden one of my accounts started showing a little triangle icon next to its mailbox. Upon clicking on this icon it informed me that the connection to the server was refused due to minimum time between mail checks violation. WTF. When I went to Connection Doctor (you can find it in the Windows menu) all was looking OK. And yet, I had messages on the server that were not being downloaded.

Searched the net far and wide looking for a solution. Found nothing helpful. Some suggested removing preferences file. Bad idea. It screws up with with your existing settings. Sets up some new accounts but that’s pretty much it. In the end what fixed it was the removal and subsequent re-creation of the offending mail account. Now it works.

So, there you go, problem solved.
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Without wax,
yOOrek