The informal email is one of the two faces of CELPIP writing task 1. Mechanically it is identical to the formal version: a short situation, three bullet instructions, about 150 to 200 words, roughly 27 minutes, typed into an on-screen editor and marked by CELPIP raters against the same four criteria. Only one variable changes, and it changes everything. The reader is someone you know well.
That single fact rewrites your vocabulary, your sentence lengths, your opening and your sign-off. "Dear Mr. Rodrigues, I am writing to request the use of your vehicle" is a perfectly good sentence and a bad answer, because the scenario said the reader was your cousin. Register sits inside Task Fulfilment, which is 25 percent of your score, so writing a polished formal email to a friend does not earn you points for effort. It costs you a quarter of the mark.
The trap runs the other way too. Candidates hear "informal" and produce a stream of slang and abbreviations, on the theory that more casual equals more informal. Raters are not looking for a performance of casualness. They are looking for the register a real adult actually uses with a real cousin: contractions, plain words, a genuine opening question, short sentences mixed with longer ones, and a closing that sounds like a person rather than a template.
And informal does not mean unplanned. This is the part candidates most often miss. Content and Coherence is 30 percent of your score and it does not get a discount because your reader is family. The same 150 to 200 words still need paragraphs, still need to be in a sensible order, and still need to cover all three bullets. A warm, friendly, structurally chaotic email lands in the middle bands just as reliably as a cold one does.