#additional combat phases

[magicjudge]

In my first main phase, I cast Neheb the Eternal. In my Combat Phase, I deal 5 damage to my opponent. In my Postcombat Mainphase, I add 5 Red to my mana pool. I use that mana to activate Aggravated Assault. In the new Combat Phase, I deal 5 damage to my opponent. Does Neheb's second ability trigger again in the new postcombat main phase?

Yes.

Any ability that triggers at the beginning of your postcombat main phase will trigger at the beginning of each postcombat main phase if there are multiple combat phases in the turn.