Can forwarding address be provided by e-mail for landlord to return security deposit?

Depends upon your goal. If your goal is the return of the security deposit, then anything that is acceptable to the landlord will work. Kiss-o-gram, smoke signals, phone calls or whatever. If the goal is to get damages, then your references are clear: paper or fax (fax numbers can be purely digital).

§88 gives an exhaustive list of legal ways to deliver the written notice. Each and every one of them involves delivery of a physical piece of paper, including fax transmission to a number provided by a recipient. Apart from direct hand-over, ordinary mail is acceptable so registered mail is not required. Email and phone-based messaging do not constitute legal notice having been given.