DyamoDB put item with ConditionExpression instead of key

Since email is not the primary key, I need to check the uniqueness of a record based on the email field. It does not work. The user gets saved. Does DynamoDB not allow conditionExpression on another field instead of a key?

    const params = {
            TableName: process.env.tableName,
            Item: user.toItem(),
            ConditionExpression: "#email <> :email",
            ExpressionAttributeNames: {
                "#email": "email",
            },
            ExpressionAttributeValues: {
                ":email": body.email,
            },
        };
    

    await docClient.put(params).promise();

The condition is valid, but what conditional puts prevent is overwriting records with the same primary key:

The PutItem operation overwrites an item with the same key (if it exists). If you want to avoid this, use a condition expression. This allows the write to proceed only if the item in question does not already have the same key.

To prevent duplicate emails, make it part of your table's primary key or manually check for uniquenes before writing to DynamoDB.