It's bluf. If you answer, you have to pay a "fine" to pay off any further steps from being taken. (You invoice is always in € the "offer" for the fine only stands for 1 day and can only be paid in bitcoins, but don't worry, a how-to is included /s )
our isolated systems has received multiple unsolicited incoming connections from an IP address under your control
Is utter bullshit. They just try to impress/scare with facy words especially by letting them make no sense (assuming the reader will think 'i dont understand, so they must be right'). You can safely Just ignore it. Delete it.
IF it makes you feel better, call your isp on the number you get from their website or from an invoice. Do NOT use any details from the email.
The three way handshake only proves an established tcp connection. Not with whom. It can still be with the man-in-the-middle.
You highlighted a few lines from the mail, but you can highlight the whole message. It's bullshit after bullshit. No isp excpects the enduser to monitor the flows. I do, but my isp's helpdesk had no clue what it is.