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So far, I've been using this sentence in my official mails whenever I send e-mail to my American clients. I want to be sure whether that my American clients really understood understand what I tried try to say.

I don't mean to pick on anyone's grammar. I'm doing this as a grammar exercise. The above seems to have a few grammatical mistakes which I have highlighted. Please correct me if I'm wrong.
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Looks ok.
I would make a couple of other changes: I don't think you need "in my official mail", and you could equally say "what I say".
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IMO:
... really understand what I'm trying to say
seems more "pro-active" to me.
See at the New York Times:
1 from nytimes.com for "understand what I try to"
567 from nytimes.com for "understand what I'm trying to"