Certified Cease Letters?

[Tia Co]

Hello,

I am new to investing and started my mail out campaigns just last month. I checked my P.O box today and had a notice of a certified letter. I hadn't checked my P.O. Box in weeks as I had planned to only stop by a few times a month to scoop up returned mail. Unfortunately the post office only holds it for a certain period and had already sent the certified letter back to the sender.

I am assuming it is a cease letter but I have no way of knowing which household sent it which makes me hesitant on sending follow-ups. My phone number is on mail outs but I am thinking of including my email address for those who do not want to call and essentially confirm their phone number. 

Has anyone been in this situation? Is it advisable to include an email address? I had omitted it because I want to encourage phone calls, but I realize this may not be everyone's preferred method of contact.