In the comments to one of the answers to this question, the asker points out that he is asked to submit his review of a manuscript for publication via email and that there is no formal online system with checkboxes or text boxes. This struck me as strange, as having reviewed papers for several biology and math journals I have never been asked to submit my review via email to the editor. Even for very small journals published independently, I have always been asked to use an online system, where all contact is viewable by the editor-in-chief and other parties at the journal.
Do reputable journals ever ask reviewers to submit their reviews via email, or is such a request a red flag that you are being asked to review for a scam journal? Do all reputable journals have online reviewer systems?