For the first, I would use a colon. " (that is). For the second, normally a signature would follow a letter closing, and the writer would be identified in the title; but in the informal world of today, e-mails don't permit as much structure.
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goronskyShould the comma below be replaced by either a colon or possibly an em dash? The first sample below could be interpreted as a direct address.•I would like to endorse a man for whom I have tremendous respect, Steve Smith.Do you mean that you are addressing Steve Smith? If you are, it would be more natural for you to begin the sentence with the name.