1. Sir, the term "refused" on 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 and in such situation, the said letter, in my opinion, can be treated as having 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 ground of non-service of 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 and I think, in e-mail also there are some features which 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 such features.