Referencing An Object's Fieldnames

A have an array of objects defined like this:

var collegas =
[{smtp:'alberic.verhelst@king.dom',voornaam:'Alberi c',famnaam:'Verhelst'},
{smtp:'anita.dierens@king.dom',voornaam:'Anita',fa mnaam:'Dierens'}, etc


I iterate over this array with this code :

for (var i = 0, stop=collegas.length ; i < stop ; i++) {
for (var veld in collegas[i]) {
etc

I would like to check whether a particular veld instance's name is smtp. In
code what I want is this :
if (veld.fieldName=="smtp") { do something ...

But this syntax doesn't work.....