I had a spam message in my inbox which was dated 03/10/44 (or something like that)
The soap call to ListMessages returned negative value for that date (logically correct since it was number of seconds from 1/1/1970, negative!)
However .net response parser fails since wsdl defines that field as Unsinged 32-bit Integer.
Any idea how to work around this bug?

Soap classes are added using Add web reference as recommended in YMail documentation.