What to include
Four lines are enough and they save a great deal of back and forth. The address of the page. The sentence or the source line the message is about, quoted rather than paraphrased. What is wrong with it. And, where the point is factual, something that settles it: a link, a document, a date. A message that contains all four is usually actioned without a reply being needed first.
Reporting a source that has closed
This is the most useful thing anybody sends, and the most common. Every source on the site carries the date it was last reached, and a date is a claim that can go stale: a page that resolved in August can be gone in November, and nothing about that failure announces itself. If an annotation describes something that no longer answers, saying so is a genuine contribution.
When a source has closed rather than moved, the entry is rewritten to say so and to keep the record, rather than having the line quietly removed. That is deliberate. The report on closed measurement tools exists because the closures turned out to be the more interesting half of the subject, and the same principle applies at the level of an individual line.
Suggesting an entry
The index records well over a hundred terms and keeps a handful as full entries. A term is promoted when it still has a real, checkable set of sources worth annotating, which is a judgement about the sources rather than about the subject. If you know of a term in the index where that is true, it is worth saying which sources you have in mind.
What this address is not for
It cannot look a domain up for you. Nothing on the site queries anything: the reference section explains how to read a registry record, a DNS record and the estimated figures, and the decoders name the fields in a record you already have, in your own browser. Running the query is something to do yourself, and the pages set out how.
Wanting the traffic is not what puts a source on a list. A source is listed because a reader would want it, described accurately, with its class and its date stated. That is the whole basis on which the catalog is worth anything.