Only 5 attachments at a time will be processed via email.
For Dropbox take a look here:
http://community.quickfile.co.uk/t/dropbox-isnt-working-what-you-need-to-check-for-first/405
EDIT:
If you’re still having problems let me know and I’ll take a look. You have a few accounts so I didn’t know which one to look at?