1. Sir, the term "refused" on the cover of the letter, as recorded by the postman or delivery clerk/official deputed for the purpose, will mean that the person to whom the letter is addressed knows the contents of the letter. In such a situation, the said letter, in my opinion, can be treated as having been served on the concerned person.
2. However, to ensure that the person (who refused to receive the letter) may not contest the case on the grounds of non-service of the letter/show cause notice, it is essential, in my opinion, that the remarks of the peon/clerk who was deputed to serve the notice are recorded properly.
3. The said show cause letter can also be served through e-mail. I think that in e-mail, there are features that can acknowledge that the sent letter has been delivered/read by the said person. Computer experts who operate e-mails in Outlook, etc., I think, may know of such features.