Both "registered mail" and "certified mail" with receipt are acceptable to prove mailing a refund claim

Maryland Court of Special Appeals issued it's decision that when mailing an amended return or refund, make sure to send paperwork to the Comptroller's Office by registered mail and keep the receipt.   Maryland court has recently stated that REGISTERED mail receipt is required to prove mailing of refund claim and CERTIFIED with return receipt mail.

Such proof is required in cases when the Comptroller's office claims to have never received the amended returns/paper refund tax returns and can not track them down.