出る is intransitive verb, it’s something that can occurred by itself. For example “The man leaves” (he decided to leave by himself!).
While 出すis a transitive verb. Someone will need to send the letter, it won’t be sent by itself! (unless it’s magical letter, lol).
Another way to distinguish them in a sentence is to check out their ます form. 出す(dasu) is 出します (dashimasu) while 出る (deru) is 出ます (demasu)!
Happy learning! (๑•̀ㅂ•́)و✧