It should be noted that the Free Tonido version has a limit of 10 "Email Send Limits" in a 24 hours period before you get an error code: Email Send Limit Exceeded.
After the 24 hours...it appears to reset...
Or you could follow the other suggestion to copy-N-paste the unique URL information and send it via an independent mail service ( like Gmail, yahoo...etc) because generally they don't have a small usage limit.
I experience this and it took a while to finally get a CLUE... as to what this error code meant and what the magic # was.
Tonido is trying to prevent their server from unwanted SPAM....that's fine....but NOW it is documented so other Tonido users will be able to find the documentation without pulling their head-hair out searching for information that DID NOT EXIST for the USERS to find.