What lesson can we learn from the outcome of the poem "A Poison Tree"?

The lesson we can learn from the outcome of this poem is that it is healthier, if we are angry, to talk to someone about it than to keep our angry feelings buried inside.
This lesson is summed up in the first stanza of the poem. Here, the speaker says that he was angry with a friend but talked to him about his anger, which made it go away. However, when he gets angry at a person he considers an enemy, he doesn't talk about it. Therefore, his anger grows. As the poem states,

I was angry with my friend;
I told my wrath, my wrath did end.
I was angry with my foe:
I told it not, my wrath did grow.

The anger continues to grow while the speaker smiles at his enemy and pretends everything is fine. In the end, the anger bears fruit, which is a poisoned apple. The poisoned apple is a symbol of a relationship poisoned by anger. The speaker gives this apple to his enemy, who eats it and dies.
The lesson is that anger, if not dealt with, will destroy a relationship.