Moq VerifySet fails when using an interface property setter
_smtpClient.VerifySet(x => x.Credentials = It.Is<NetworkCredential>(c => c.UserName == smtpuser && c.Password == smtppwd));

Assert.IsNotNull(_smtpClient.Credentials);
Assert.AreEqual(_smtpClient.Credentials.UserName, smtpuser);
Assert.AreEqual(_smtpClient.Credentials.Password, smtppwd);