Yeah man, I'm the mailman, can't you tell, man?

from Kendrick Lamar – Fuck Your Ethnicity Lyrics on Rap Genius

Meaning

Kendrick is the deliverer of messages to the people and therefore is a proverbial mailman.

Could be referencing former NBA star Karl Malone who was known as the Mailman. Malone was a Power Foward (a “post player”) so which could tie into the next line of “going postal

Mail is also slang for money.

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