Letter to a colleague, Formal or Informal?

Question

Recently I have got a question as below:

Write a letter to one of your former colleagues about your upcoming visit to the city where you both worked together. In this letter please include:

-Your current life condition -Detailed plan of your visit -Meeting arrangments with him

How should I approach this letter? in formal or informal way? Because I am writing a letter to one out of at least a hundred colleagues. That must make him one of my close ones. So, in my opinion, informal tone seemed appropriate.

Answer 1

Unless it's someone you still regularly correspond with, I'd go for somewhat informal.

Formality/informality isn't a matter of distinct grammatical registers in English, as it is in some languages. It's a spectrum. You can be excruciatingly formal, completely informal, and many points in between. This would want to be less formal than a business letter, but probably not all chummy, the way you talk to friends in the pub sort of informal.

Unless, of course, you still regularly correspond with the person. Then is should whatever level of formality that correspondence is usually in.