I’ve tried Balsa with iCloud, it connects and downloads emails but with severe limitations such as the inability to store the password. Balsa keeps asking it every time is started.
I know this is not an answer to your question but maybe you can give Claws Mail a try and see if it suits your needs best.
I know people from itHUG who are successfully using it with Gmail and there’s also an experimental plug-in for Haiku.
I hope this helps.
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