For 15+ years, we’ve seen every kind of “sync failed” error between Mailchimp and Salesforce — the kind that shows up at 5 PM on a Friday right before a campaign send.

If your integration suddenly stops working, here’s the truth: it’s rarely Mailchimp or Salesforce alone. It’s usually the way the two talk to each other — or stop talking.

Let’s break down what’s really happening and how to fix it fast.

Step 1: Understand What Broke (It’s Usually the Token or the Data)

The most common causes:

OAuth Expiration: Your Mailchimp token expired and wasn’t refreshed by the connector. This happens silently — until your next sync.
Custom Field Mismatch: You renamed or deleted a field in Salesforce that the integration depends on (like Email or Lead Source).
Campaign ID Errors: Salesforce Campaigns synced before, but new ones aren’t mapping because of permissions or API changes.
Duplicate Contacts: When both Mailchimp and Salesforce try to “own” the same record, one wins and one loses — usually the wrong one.

Pro Tip:
If you’re syncing via API or middleware, run a test sync in “update-only” mode first. That isolates if the break is schema-based (fields) or credentials-based (OAuth). These features exist in each Salesforce SyncApps integration in Step 1 & 2.

If you’re wondering why integrating Salesforce and Mailchimp matters, we’ve covered the key reasons most businesses rely on the sync daily.

Step 2: Fix the Connection — Don’t Rebuild It

Rebuilding your integration is the fastest way to lose historical data. Instead:

Reauthorize Mailchimp in your integration tool or connected app settings in Salesforce.
Run a small sync (10–20 records) to confirm token validity.
Re-map custom fields only if you’ve changed the structure since your last successful sync.
Disable duplicate check rules temporarily — test a full bidirectional sync, then re-enable once you confirm it’s flowing.

If you’re using SyncApps, all of these steps can be done in your Integration Settings → Step 1 & 3, and the logs will show why the sync stopped. If you’ve never set up your integration manually, check out our guide on how to connect Salesforce to Mailchimp step-by-step.

Also if you need to resubscribe unsubscribed records safely, here’s the exact method used by Salesforce admins to stay compliant.

Step 3: Automate the Fix Before It Happens Again

Once your sync works again, lock it down:

Set Token Auto-Refresh (SyncApps does this automatically).
Add Email Alerts for Sync Errors so you catch issues before your team does.
Use Bidirectional Field Mapping to prevent overwriting clean data with stale lists.
Schedule Daily Syncs so you’re never running huge bulk updates again.

The goal isn’t just to fix the sync — it’s to make sure it never breaks the same way twice.

Want to keep Mailchimp groups and Salesforce segments in sync automatically? This guide walks through the setup.

Bonus: A Quick Way to Avoid All This

If you’re still managing your integration through manual exports, one-off scripts, or a “native” connector that doesn’t understand your CRM schema — you’ll keep fighting these fires.

SyncApps automates the entire Mailchimp–Salesforce sync, with:

Token auto-renewal
Real-time subscriber updates
Campaign ROI tracking inside Salesforce
Full bidirectional support for custom fields and objects

See how it works in our Mailchimp + Salesforce demo

Because the best sync is the one you never have to think about.

We’ve helped thousands of Salesforce and Mailchimp users fix their syncs since 2011.
If your sync just broke again, you’re in good company — and we can help.

All Mailchimp + Salesforce features are included in the SyncApps One Plan pricing, with no per-connector fees. One Subscription for all your automation needs.