It seems that settings of mail accounts have something to do with it. I've delete my gmail account and am now using Microsoft Exchange. It seems better, yet not perfect. The issue is definitely due to OS 4.3.1
Apple answer is - no answer. It is a shame that the quality of their products, in terms of design and concept, is not match by a care for customers. It is not normal that I, as a customer, have to spend hours reading Apple Discussions forum to try to understand what the problem is. A little more respect for a customer that spend big $$$ buying their products wouldn't hurt.