Right now the "Best" alternative Email Client Alternative to Outlook

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I do Enterprise support to 259 people out of a 2800 seat situation at a large government institution. I am the type of IT/support person who believes that just sometimes you have to go outside of the normal box....Thunderbird is...well okay, I have users that do not want to re-learn a whole lot, they do allot. eM Client is different, but not a pain to discourage to re-learn and understand, after that, the praises are totally about how fast it is at launch and for search. Thank you for your quallity :-)