Is it possible to Email someone with a different Email Service?

Say for Example, I am a Gmail but the person I want to Email has a Yahoo, Would it be possible to Email to them using my Gmail?

That is not to say I dont have a Yahoo (Since How would I have asked this question if I didnt) It just makes it more Convenient to email someone with Gmail from my Phone.
Update: Thank you so much for all the Answers, It will be Very Difficult to give someone "The Best Answer" When the option becomes available for the Question, since all of you have done an Amazing job.
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