SendOnBehalfOfName strange problem

I am trying to use SentOnBehalfOfName to set the "from" address.
The problem is that I use a combo to have users select the from address from
a list. I change the SentOnBehalfOfName every time users change selection on
the combo. Seems that only the frist time the "From" is really changed. I
also put the from recipient text on the form to check it and tha strange
thing is that the text show the actual content of the SentOnBehalfOfName but
the real from does not change. If I change manually every work fine.
I setted up a combo with 2 address for tests, the first one is enabled as
send as for the user tha second one not. If I select the second one and try
to send I receive back, correctely, a message stating that I dont have a send
on behalf right; but when I try to select the first one I continue to recieve
that message.
Obviously If I select the first one as soon as I created a new mail from
that custom form and send the mail every work fine.
Could anybody help me cause I dont want to suppose that users cannot change
their mind in selecting the combo :-)

thanks