ISD asks that letters from a business or organisation are on official headed paper showing its full name, postal address, fixed-line phone number, website, email address and a named contact person with their title. A translation must reproduce all of those details.
ISD verifies these letters, which is why it's particular about the details: a mobile number won't do, and nor will a Yahoo or Hotmail address (ISD has firm views on webmail). We translate the letterhead as well as the body, so the verification details are readable in English and nothing on the original is left out.
If the letterhead is a logo with a tiny address line in the corner, we translate that too. It's the part ISD wants to check.