mc_cid and mc_eid are added by Mailchimp to links inside email campaigns. mc_cid identifies the campaign, meaning the specific newsletter that was sent. mc_eid identifies the individual recipient, meaning you.
That second one is the reason this post exists. Most tracking parameters identify a click or a campaign. This one identifies a person, and it travels in plain sight in a URL that people forward without thinking.
What each one does
A link inside a Mailchimp newsletter arrives looking like this:
https://example.com/article?mc_cid=8f2a1b9c3d&mc_eid=7e4d2c1a9b
mc_cid is the campaign identifier. Everyone who receives that newsletter gets the same value. It answers the question "which send did this click come from".
mc_eid is the subscriber identifier. It is different for every recipient. It answers the question "which person clicked".
Together they let the sender see that a specific subscriber opened a specific campaign and clicked a specific link. That is standard email marketing and it is what most senders expect their platform to do.
Why this one is different from the rest
Every other parameter in this series is generated at click time and identifies an event. mc_eid is stable. It is attached to you as a subscriber to that list, and it stays the same across campaigns.
Now think about what happens when you forward the link.
You read a newsletter, find an article worth sharing, copy the URL out of the email, and paste it into a group chat or a Slack channel or a public forum. The mc_eid goes with it. Anyone who clicks that link registers, in the sender's analytics, as you.
Post it publicly and it is worse in a quiet way. The identifier now sits on a page that anyone can read, permanently associated with whichever list you subscribed to.
I do not think most people forwarding a newsletter link have ever considered this, and I include myself for a long stretch. I had been cleaning utm_source out of habit for years before I properly looked at what mc_eid was doing, and I found it a genuinely uncomfortable realisation. It is the one parameter where the honest answer to "does this identify me" is closer to yes than no.
What it does not do
Worth being precise here, because this topic attracts overstatement.
The value is opaque. Somebody reading the URL cannot decode your email address from it. It is a key into the sender's list, not an encoded address.
What it does mean is that the sender, and anyone with access to that Mailchimp account, can resolve it back to a subscriber record. That is not the general public. It is a specific, small group of people. Whether that matters depends entirely on the list. A newsletter about kitchen renovations, probably not. A newsletter from a support organisation or an advocacy group, possibly quite a lot.
Other email platforms do this too
Mailchimp gets named because it is the biggest, not because it is unusual.
HubSpot uses _hsenc and _hsmi. Marketo uses mkt_tok. Various smaller platforms use vero_id, oly_enc_id and similar. All of them encode some combination of campaign and recipient, and all of them travel in forwarded links exactly the same way.
If you are cleaning links out of habit, email parameters deserve more attention than advertising ones, not less.
Is it safe to remove?
Yes. No page needs either parameter to load. The only effect is that the newsletter sender loses the click attribution for that visit.
If you actively want a small publisher to know their newsletter drove your click, leave it. That is a reasonable choice and I make it sometimes. Just make it deliberately rather than by accident, and do not make it on behalf of forty people in a group chat.
Removing them
Both parameters together, by hand:
https://example.com/article?mc_cid=8f2a1b9c3d&mc_eid=7e4d2c1a9b
https://example.com/article
Pasting into the url cleaner tool online strips them and leaves anything the page genuinely needs.
The habit worth building is cleaning the link before you forward it, not after. Once it is in a group chat it has already been distributed.
Related reading
The readable, human-written parameters that often sit alongside these are covered in UTM parameters explained.
For the advertising equivalents, see what is fbclid.
Everything in one place: the URL tracking parameter glossary.
Frequently asked questions
Can someone get my email address from mc_eid?
Not by reading it. The sender can resolve it to your subscriber record on their own account.
Does mc_eid change between campaigns?
It is tied to your subscription rather than to a single send, so it is considerably more stable than a click identifier.
What happens if I forward the link?
Every click on the forwarded link is attributed to you in the sender's reporting.
Should I stop clicking links in newsletters?
No. Just clean them before sharing them onward. That is where the actual leak happens.
Does this apply to the unsubscribe link too?
Unsubscribe links contain their own identifiers by necessity, since the system has to know who is unsubscribing. Never share one.