Here's a static method design that allows method mocking/verification via Daniel Cazzulino here http://blogs.clariusconsulting.net/kzu/making-extension-methods-amenable-to-mocking/.

ClassWithStaticMethodFactory.cs

public static class MessagingExtensions
{
    public static IMessaging Messaging(this SomeType target)
    {
        return MessagingFactory(target);
    }

    static MessagingExtensions()
    {
        MessagingFactory = st => new Messaging(st);
    }

    internal static Func<SomeType> MessagingFactory { get; set; }
}

StaticMethodTests.cs

[TestClass]
public class StaticMethodTests
{
    [TestMethod]
    public void Can_mock_static_method()
    {
        var mockMessaging = new Mock();
        MessagingExtensions.MessagingFactory = st => mockMessaging.Object;
    }
}