In order to create issues via email, you'll need to find the correct email address for the desired project. You can find these email addresses by going to Settings then selecting the Account tab. From here, click on "Log Issues by Email" in the sidebar. You'll see a list of projects and email addresses that you can use to create issues.
These email addresses are unique to each project so that Sifter knows where your emailed issues belong. Also, these email addresses will only accept email from the address associated with your Sifter account. If you submit issues from another email address that Sifter doesn't recognize, you'll receive an error message via email explaining that the issue was rejected because Sifter didn't recognize the "From" address.