Absolutely. You can forward emails to create issues, and Sifter will do a few things to make sure they're as readable as possible.
The key to forwarding emails is to leave the "Fwd:", "FW:", or other similar text at the beginning of your email subject. (Don't worry, Sifter will automatically strip this part when creating your issue's subject.)
This way, Sifter knows to include anything you write as plain text, and it will automatically included the forwarded content as quoted text.