I have the same issue and I am sure microsoft is actively blocking the email to outlook. You can add any safe sender except the one from dropbox and if I send the confirmation email to my work address, it all works fine. Shame on you microsoft!!! I am sure this is a violation of your powers (cfr google being convicted in europe for blocking adds from rivals). I hope dropbox reports this!
[This thread is now closed by moderators due to inactivity. If you're experiencing a similar behavior, feel free to start a new discussion in the Dropbox Community here.]