c# find outlook contact by email address

Question

I was able to find outlook conctacts by firstname and lastname this way:

Outlook.ContactItem contact = (Outlook.ContactItem)contactItems.Find(String.Format("[FirstName]='{0}' or "+ "[LastName]='{0}'", namePart));

Now since I am looking for a contact via emailaddress I thought this should work, but it doesn't:

Outlook.ContactItem contact =(Outlook.ContactItem)contactItems.Find(String.Format("[Email1Address]='{0}'", namePart));

But unfortunately it won't find the contact, there is, with the email address passed to the method and filled into namePart.

Can anyone tell me, where my mistake lies? Thanks in advance!