Your Email Platform's Default UTMs Are Breaking GA4

The auto-tagging you never reviewed

Somewhere in your email platform's settings is a toggle called something like "Google Analytics tracking." Someone turned it on years ago, probably during onboarding, and nobody has looked at it since. That toggle decides what utm_source and utm_medium every email link carries. And each platform made different choices.

The result shows up in GA4 as a fractured email channel. Some email traffic under Email, some under Unassigned, some split across sources like "klaviyo" and "hs_email" and your actual sending domain. Same channel, three reports. When email revenue looks weirdly low, this is one of the first places we look.

What each platform actually sends

The defaults, as of when we audited them:

Klaviyo tags campaigns and flows differently out of the box, and its default medium values haven't always matched GA4's channel rules. Klaviyo also changed its default utm_medium behavior in mid-2025, which quietly invalidated a lot of older setup guides. If you configured Klaviyo UTMs before then and haven't re-checked, your flows and campaigns may be reporting under different mediums right now. Their community forum is full of threads about email and SMS traffic blending together because both used the same medium.

Mailchimp appends its own tracking parameters (mc_cid, mc_eid) on top of UTMs, and its UTM defaults derive utm_campaign from your campaign title. Rename a campaign draft five minutes before sending and your GA4 campaign name reflects whatever the title happened to be. Auto-derived values feel convenient until you try to compare "March_newsletter_FINAL_v2" across months.

HubSpot sets its own source and medium values that don't match what your other platforms call email. At least it did last time we audited it, and that hedge is kind of the point. Defaults change without announcements. Trust nothing you haven't clicked and inspected this quarter.

The specifics matter less than the pattern: three platforms, three opinions about what email traffic should be called, zero coordination with your conventions.

The audit takes ten minutes

Don't read docs. Docs describe intentions. Inspect reality:

1. Send yourself a test from each platform (campaign AND automated flow, they often differ).
2. Click every link type: body link, button, footer.
3. Look at the URL in your address bar. Write down the exact utm_source, utm_medium, utm_campaign.
4. Check the values against GA4's channel rules. Anything that isn't utm_medium=email (or a close variant) is leaking out of your Email channel.

We ran this exact audit for a team last year that swore their email program was fully tagged. Their welcome flow, running for three years, had no UTMs at all. Three years of their best-converting automation counted as direct traffic.

Set explicit values everywhere

The fix is boring and takes an afternoon. In every platform, turn OFF auto-derived values and set explicit ones:

• utm_source: your list or platform identity, spelled one way. We like the platform name (klaviyo, mailchimp) because it tells you which system sent it when something breaks.
• utm_medium: email. Exactly that. Every platform, every message type. Your SMS sends get utm_medium=sms so the channels separate cleanly.
• utm_campaign: set per campaign, following your naming convention. Never auto-derived from a title someone typed at 11pm.
• utm_content: the one auto-derived value worth keeping, for link position or variant if your platform supports it.

Then put the convention in your link builder so newsletters assembled by hand follow the same rules as platform sends. Mixed manual-and-automated email programs drift apart fastest, and the governance problem is the same one every other channel has.

One thing this won't fix: open rates. Those died with Apple's Mail Privacy Protection and they're not coming back. Click data is the email metric that still means something, which is exactly why the tagging on those clicks deserves this level of care.