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i've been using EL for near a decade, I dont believe they do, if they have implemented it, i've not heard about it.

sorry
 
Just as a word of warning, I find that the IMAP support in Mail is lacking. I had problems retrieving new messages. Sometimes I had to quit and reopen Mail to get any messages that had been sent since last opening Mail. Hope that made sense 🙂

Of course, that doesn't answer your question. I have no idea about what any American ISPs support.
 
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