Email and landing page copy fail differently, and the usual mistake is running the same letter in both places.
The reader arrives in a different state
An email reaches someone who did not ask for it at that moment, so the opening has to earn attention against everything else in the inbox. A landing page receives someone who clicked toward it, already partly qualified. Copy that opens by establishing relevance is essential in one and wastes the reader's momentum in the other.
Length is governed by the medium, not by doctrine
Email competes with an inbox and rewards brevity with a single clear action. A landing page competes with the back button and can run long, because a reader who is genuinely evaluating will keep reading. Applying long-form email discipline to a landing page, or the reverse, reliably underperforms.
They should not carry the same job
The email's job is usually to earn the click, not the purchase. Trying to close entirely within the email duplicates the landing page and gives the reader two chances to decide against it.
Test them separately
Subject line and opening drive email performance; headline and offer framing drive landing page performance. A change that improves one frequently does nothing for the other, so shared test results are misleading.