[elb2020]

"It looks like you forgot to do X; that's needed to do Y."

"It looks like you forgot to do X. That's needed to do Y."

I would actually prefer the second one here. It's neutral and to the point.

I guess the problem with the first one is that the reader might interpret the semicolon exactly in the way that you intended it, as an attempt on your behalf to "control the tone". Depending on the personal disposition of the receiver he or she may or may not like it.

[ajkjk]

Oh, weird, I find the second off-putting. It's a bit.. Patronizing? Like you're some superior giving orders.

[emmelaich]

There's a lesson here; never try to infer too much tone from email messages.

Or chat channels. Unless you really know the person well.

[Spivak]

Unfortunately there’s a bit of knowing your audience because you can’t really control the tone of voice inside your recipients head. To anyone who’s Millennial or younger the one with the period is straight up aggressive.

For generations that grew up as text as the primary form of communication we had to figure out how to convey tone and writing style and emojis is what we had. Tone and the expression of tone are disconnected even when speaking in person and have grammar like rules.

[elb2020]

And the corollary; never try to insert too much tone into the same.