Depending on the service you use the solution may vary. Typically there are four measures that may help:

1) use the specific email address of the cloud service dropbox (if it is a unique address this should be enough).

2) if you need to verify your email address of your cloud service, gtd system or dropbox acquire a unique email address from Braintoss for sending notes, so you can approve/validate that email address if needed. This is done by sending a text note the applicable address (e.g. your default address) with the command set from unique. Now emails will be sent from a unique address rather than from, which is the email address that can be added in the other task service. 

Please note that this is a beta feature and has not been tested by support for all platforms! More detail on how to configure your cloud service can be found in this example with Wunderlist.

3) it often helps to change the Braintoss email sending settings to plain, which removes the prefix, the signature and the HTML.
To do this, send a Braintoss note with the text SET PLAIN to the dropbox email address. (see How do I change the format of the email? for more options)

For Evernote send a Braintoss note with the text SET EVERNOTE (to your eernote address)

NOTE for Evernote users: For Evernote to be able to send emails to your account you need a Premium subscription!

*For OneNote look at Dave Dirk's blog or on the website

*More information on MeisterTask see this article on their website

*For Checkvist look on the checkvistpage.