Thanks, your point on e-mail address of collection account is right: I've set a dummy address for that and then set the right one for the SMTP account.
I find this not much intuitive, but it works well.
Take care that I use different server for incoming and outgoing, but the outgoing one is SPF authorized, so I'm not having any security/spam issues. I think that many other people could have the same requirement (a different SMTP server/authentication from the incoming/POP ones).
Thanks for the help.