Troubleshooting List Imports

MailChimp scans each list import file for duplicates, bounces, and unsubscribes, as well as errors. Incorrect file formats, incomplete or malformed email addresses, or other inconsistencies in the file, can cause some or all email addresses to be blocked from import.

In this article, you'll learn more about the issues that could cause problems with your import, and how to undo a list import.

Problems with File

If MailChimp won't accept your file for upload at all, there are a few possible causes.

Unsupported File Type

If you chose the Import from a CSV or TXT File option, make sure the file you upload is a CSV file or tab-delimited text file. The system does not support of Excel (.xls), ACT!, .vcf (vCard), or other file types.

Switching ESPs

  • Exported files from other ESPs may not be formatted in a way that our import tool can manage, so take some time to review our formatting guidelines.
  • Our groups feature only supports 60 groups. If you switch from Constant Contact with 60 groups or more, you may see an error on your import.


You can import OPTIN_TIME and CONFIRM_TIME fields, but they are only checked the first time you import a contact and can't be overwritten later. Also, future dates in these fields will cause an error, so double check that this field data is correct.

Syntax Errors

Malformed or incomplete email addresses are treated as syntax errors, and won't be added to your list. Other syntax errors we remove from your import include unsubscribed and duplicate email addresses. We'll display the number of errors found on the Import History page when the upload is complete.

To fix these addresses, you can view which addresses were removed and add them back individually or as a new import. Click the X emails had syntax errors link on the confirmation screen, or visit the Import History page to view the import history. Upload details are viewable in the account for about 24 hours.

Here are some common causes of syntax errors.

Missing Information

An address could be missing something. For example, john@aol is missing ".com," and john@aolcom is missing the dot. After the address has been corrected to, it can be uploaded to the system.

Spaces in the File

An address may include unnecessary spaces, before, after, or even in the middle of the address because of how some export utilities work. These extra spaces can cause an otherwise valid address to be reported as a syntax error. Try the TRIM function in Excel to remove spaces from text, or search your help documentation for a similar feature in your preferred spreadsheet application.

Typos in the Address

Look for typos. MailChimp scans for misspellings of common domains and removes them from your import to prevent bounces. For example, we'll block "" or "" because Gmail and Yahoo were misspelled. Once you fix the spelling, you'll be able to import the addresses.

Unsupported Characters in the Address

MailChimp can't process addresses with UTF-8 encoded characters in the email prefix, which is the text before the @ symbol. Check your list for special characters in the email prefix.

Semicolons Instead of Commas in the File

Some European spreadsheet programs use semicolons rather than commas to separate data fields in CSV or TXT files. This can cause problems with MailChimp list imports. If your import file has semicolons, replace them with commas and try your import again.

There are two ways to remove semicolons quickly. One option is to upload your file to Google Drive, and save it as a CSV. Or open your file with a text editor, and perform a find and replace search to replace the semicolons with commas.

Subscriber Count Doesn't Match

If the numbers on your Import History page don't match what you expected to see, based on your subscriber count, here are two things to look for.

Auto-Update Feature

When Auto-Update is turned on, we add opted-in addresses as new subscribers and update existing subscribers' profiles with the new information you import. If an email address appears multiple times in your list import file, we'll import it once as a new subscriber and update that profile with the information in the subsequent rows. For example, if you upload 10 addresses, where a single subscriber's information is included on three rows, your total subscriber count would only be eight.

If Auto-Update was turned on, your list import results will read that "X email addresses were updated or added," instead of just "added."

Role-Based Addresses

Email addresses defined by a job or title rather than a person are known as role-based addresses. Typically, these addresses are managed by several people and sometimes fall into disuse. This can lead to high bounce rates and spam complaints, so MailChimp will automatically remove them from your import. You can still add subscribers with role-based addresses to your list, but you'll need to manually add them or request they subscribe via your list's signup form.

Import Size is Large

Lists of more than a hundred thousand contacts can take some time to import. We'll send a confirmation message to the Primary Account Contact's email address, and post a notification in your Account drop-down menu when the import is complete.

File Size Limit

Imports from CSV or TXT files have a maximum file size of 200MB, or about one million contacts. CSV or tab-delimited text files larger than 200MB can cause import errors. 

If you experience an error when importing a big file, split up your file, and do several smaller imports.

Undo a List Import

If you make a mistake with your list import, you can undo your recent list import for any reason, including a compliance issue. This action will remove any new contacts that were added to your list in the recent import, but won't remove contacts who already existed on the list or revert any field data that was updated for existing contacts as a result of the import.

To undo your list import, follow these steps.

  1. Navigate to the Lists page.
  2. Click the drop-down menu next to the list you want to work with, and choose Manage contacts.
  3. Click Import History.
  4. Click Undo for the list import you want to remove.
  5. In the Undo Import pop-up modal, type UNDO, and click Undo.

After you confirm the action, the list import appears as undone on Import History page and displays the time and date that the import was undone.

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