What is mc_eid?

Who adds it

Mailchimp adds it to links in campaign emails when e-commerce tracking is on, together with mc_cid.

What it means

The visit came from a Mailchimp email, and the value is a unique ID for the recipient's email address. Through Mailchimp's API it can be looked up to find that subscriber.

What to do with it

It identifies a person, so keep it out of reports you share, and be careful when the reader forwards or shares the URL. GA4's data redaction can remove named query parameters before collection.

How Vireo shows it

Vireo doesn't store click IDs, in the free plugin or in Pro.

In the free plugin, Vireo drops the query string from page addresses, keeping only a few WordPress ones such as p and page_id, so a tagged URL counts as the same page as the untagged one. The visit is counted under the referrer it arrived with.

See Referrers and sources in the docs.

Sources

The mc_eid parameter is a unique Mailchimp ID that identifies the recipient’s email address, which you can use to associate orders with a contact.

E-Commerce - Mailchimp Developer

The data-redaction feature helps to prevent the inadvertent collection of PII in the form of email addresses and URL query parameters.

[GA4] Data redaction - Analytics Help