Thank you: in your case was the correspondence for the sponsor or principal applicant?
I asked the agent and she confirmed that the correspondence to sponsor will be via email (preferred method in most cases, if not then via mail).
Correspondence to Principal Applicant will be via MyCIC if you have linked your application.
Just to provide an update: i called CIC again and the agent provided me details about the letter, it was addressed to sponsor asking for the proof of relationship with principal applicant. They are asking for birth certificate.