Not receiving one-time security code in email

I am the administrator for our Dropbox business account. I have created a new user in the admin profile and sent an invite to the new user. I have set up Dropbox on an iPad that I am preparing for the new user. I have also set up his email account on the iPad. I have received the invitation on the iPad and have set up a Dropbox user account for the new user. When I log in on the iPad, I enter his email address and password. A window then appear asking me to enter the security code. It states that the code has been sent to the email account for the user.  The problem is that the code does not appear in email.  I have checked the spam folder and there is nothing there. The address is obviously correct, as Dropbox successfully sent the original invitation to this address. I have set up Dropbox without this issue on several other iPad for new users to our organisation previously, so it is unlikely that the service provider is blocking the email. I have tried accessing the help on line, but when I attempt to enter the information, it is rejected, claiming that the credentials I have entered do not match. As I have not yet been able to use Dropbox, obviously, I am not able to provide details of files shared, computers last logged in on, etc. Where do I go next?