[Spiritus]

Why aren't they using dropbox.com instead of the suspicious looking dropboxmail.com?

[inertial]

One reason could be to not let spam reports on dropboxmail.com affect the primary domain's reputation (which is also used for corporate communication). If for any reason, the reputation of dropboxmail.com is compromised, they can move on to dropboxmail-1.com etc.

[wodenokoto]

Facebook does/did the same. Must be some reason for it.