ForEach() is defined on the List<T> class, but not on IQueryable<T> or IEnumerable<T>. You have two choices in those cases:

ToList first

The enumeration (or query) will be evaluated, copying the results into a new list or calling the database. The method is then called on each item.

IEnumerable<Customer> customers = new List<Customer>();

customers.ToList().ForEach(c => c.SendEmail());

This method has obvious memory usage overhead, as an intermediate list is created.

Extension method

Write an extension method:

public static void ForEach<T>(this IEnumerable<T> enumeration, Action<T> action)
{
    foreach(T item in enumeration)
    {
        action(item);
    }
}

Use:

IEnumerable<Customer> customers = new List<Customer>();

customers.ForEach(c => c.SendEmail());

Caution: The Framework's LINQ methods have been designed with the intention of being pure, which means they do not produce side effects. The ForEach method's only purpose is to produce side effects, and deviates from the other methods in this aspect. You may consider just using a plain foreach loop instead.

public class Customer {
   public void SendEmail()
   {
       // Sending email code here
   }
}

List<Customer> customers = new List<Customer>();

customers.Add(new Customer());
customers.Add(new Customer());

customers.ForEach(c => c.SendEmail());

Remarks

Use it at all?

You might argue that the intention of the .NET framework is that queries do not have any side effects and the ForEach method is by definition causing a side effect. You might find your code more maintainable and easier to test if you use a plain foreach instead.