Constructor to transform an Outlook contact into a Dynamics CRM contact
You could create a helper function with a loop:

public string getFirstNonNullOrEmpty(params string[] names) {
    foreach (string name in names) {
        if (String.IsNullOrEmpty(name)) {
            continue;
        }
        return name;
    }
    return @"Naam niet bekend";
}


Usage:

LastName = getFirstNonNullOrEmpty(contactItem.LastName, contactItem.FirstName, 
    contactItem.FullName, contactItem.CompanyName, contactItem.Email1Address);


You might also need this because of the else branch:

if (String.IsNullOrEmpty(contactItem.LastName)) 
    &amp;&amp; !String.IsNullOrEmpty(contactItem.FirstName)) {
    FirstName = "";
}

You might want to use IsNullOrWhiteSpace instead of IsNullOrEmpty.


(I'm not too familiar with C# and don't have a C# compiler right now so the syntax might be invalid.)