Fax a mail with 101 attachments is the most obvious case.
Fax a mail with a single attachment that is too big.
Fax a blank email with a single attachment.
Fax an email with a blank or badly formed attachment.
Fax an email with an attachment that constitutes a potential security risk, e.g. an executable
Fax an email with some badly formed or potentially threatening attachments, and other good attachments. Confirm the good attachments arrive.
Fax an email with attachments and interrupt transmission before completion
You might get more guidence from a requirements spec if you have one.
I am not sure if you are looking for negative test cases or any non-functional test cases. Typically, negative tests are run against a feature than something you have given above. The scenario you have given is almost a test case than a scenario since it is very specific and short.
If you are looking for negative test cases, you have to find operations that are going to fail. The use of negative test cases is to test the application's ability to report/recover from error situations. First think of the conditions that will make the operation impossible. For example,
- Attach files and make them unavailable when it tries to transfer (assuming it is not caching files). You can use a file from a CD or a USB and remove it before transferring the file. You can delete the file too, if it is allowed.
-Use a large file via a slow connection, so that it is never going to complete. Best way to handle that is to time out the operation than waiting for ever.
-If the network goes down, it should resend the message with correct files again. If you remove the network cable or disable the network while it's transferring files, you can create that condition.
-The destination system should have enough space to store all the files. If it is not, system should report it. However, this may not be up to your application.
-force the system to reject the email at receiving end. (you can do it by changing the allowed attachment size). Your system should not try sending it repeatedly.
You can see the 100 emails is not part of these negative tests. Very likely, 100 or 10 attachemtns would not cause a change for above negative tests. Then they are negative tests for the feature than the above scenario. The problem with negative testing is you can do crazy stuff too, and you have to be careful not to cross the line between constructive tests and crazy scenarios.
If you are looking for generic non-functional tests, you can use tests like,
-Use very large files and very small files, compare the performance.
-Send multiple emails with 100 attachments and see how it handles them, and other suggestions given in the thread.
Use 0 byte files and see if it handles it correctly.(this is a functional test)
Hope this helps,