You send a campaign. Sales spike. And GA4 shrugs — the visits arrive unlabelled, the revenue gets filed under Direct, and email gets no credit. So the budget flows to the channels that measure well, not the ones that actually sell.
Untagged email traffic is invisible email traffic. The fix has existed for years — UTM parameters — but stamping five parameters onto every link in every send, by hand, without typos, forever? Nobody keeps that up. Software should do it.
(direct) / (none)
where untagged email traffic ends up in GA4
5 × every link
the parameters you'd otherwise type by hand