"Das ist mein Post" is wrong?
Post is feminine and is "post" in the sense of "post office". Posten is masculine and is "post" in the sense of a job/position (plural die Posten).
Ah, I see. Thank you. I looked up "der Post" and found it--but now I see that all the listings involve a dative construction (e.g., "mit der Post senden." Thanks.