Maryland Code, Environment § 4-210

(a) Any complaint, order, notice, or other instrument issued by the Department under this subtitle may be served on the person to whom it is directed in accordance with § 1-204 of this article.

(b) If service is made by certified mail, return receipt requested, bearing a postmark from the United States Postal Service, the person who mails the document shall file with the Department verified proof of mailing.

(c) Any notice that requires filing of a report, attendance at a hearing, or both shall be served at least 10 days before the earlier of: