@Kanky.................PD did say that Mayur will be friends, before the letter appears again.
I think the letter is a very essential thing. Its not about Mayur's friendship, but its about making Mayank realise not to jump to conclusions and make judgements about others before giving them a benefit of doubt.
and also, dont kill your curiousity and read a letter as soon as you receive it. Okay, kidding, but the letter will help Mayank become less prejudiced