"Send AS" in OWA users

Hi,

I've setup an OWA GUI for my client company, my problem is that the CEO wants to "send as" other e-mail account. the "new mail" box appears and the "other e-mail option" works fine.

now all that he has to do is just send the mail but.... he gets a permission error like this one on the picture.

i remind you all that trying to solve it:

* the work around this company is set only to OWA, no outlook at all !!!

* the plan is P1

thanks

Roy F.