I agree the corbyloon but if it is illegal then the email providers should at least make the users aware of the fact; and the parallel with gun sellers is not true if people need a licence which (which I think they do).
Anyway, I agree that it is a breach if done without permission and the email contains confidential info (even if it's just including their email address as a possible sufferer). However, the question seems to be asking is it unlawful per se, and the answer is no in that it can be used lawfully.
It would be interesting to see though what constitutes a lawful use. I have been copied in on mails in circumstance where I am not aware I have given permission for my email address to be published in this way.
If those are unlawful does that mean it's also illegal to pass on someone's email address if you haven't had permission- eg if I pass on a friend's email address to another friend?