You should write, 'As I already mentioned in the following email' if you're feeling smug. It feels good to type that on a mechanical keyboard.

I've always been partial to the "bitch, can you not read" variant, "As per my previous email", with a few "see below"s and highlights where necessary in the original email. Tends to get them on the right track. Especially if they were stupid enough to CC half of the management chain for either of you.

My favorite was the day the office golden child tried that because he was feeling froggy (he also didn't like me because I didn't put up with his office shenanigans), and he got a "we will discuss this offline" from his direct manager after one of those with plenty of documentation that the fucker was just sandbagging. Asshole was smart enough to never try that shit with me again though!