Hi J-F,
We've heard about this problem a few times now. It's actually been a really tough one for us to solve, mainly because it's usually a client-side problem, and we haven't had much luck reproducing the problem in-house. What often helps is if you just clear your browser cache (in IE, you can go to Tools-> Internet Options... and delete your temporary internet files and cookies). Then, just close all your browser windows and try again. If this doesn't work, you can also try in a different browser like Firefox (or IE if you're already using Firefox). If you're still having the problem, it'd be great if you could give us some details about your setup. What OS and browser are you using? Do you have any special browser settings? etc.
With regards to your other question, you can currently only send pay-to-view mails to other ether users. The main reason for this is that having users logged in on the ether site allows us to control access to the content, and more rigorously protect you from fraudulent activity. We are planning some future improvements to allow our mail system to interact more seamlessly with external e-mail addresses. Alternatively, if you have digital content that's ready to sell, you can create Pay-to-View buttons for your content. We give you the link and the html for your button, and you can always just paste that link or button in your normal e-mails or on your website. Then, anyone who clicks on the button will be asked to sign in or register, and then they can access your pay-to-view content (of course, they'll only see your description and subject until they pay the amount you specify).
I hope this helps, and feel free to post any other questions.
-Kelvin