Defines the e-mail address of recipient(s). These are seperated by ; and an e-mail adress can either look like email@example.com or My name <firstname.lastname@example.org>. Note that HdrTo is not used to actually send the e-mail, HdrTo is used for e-mail clients to visually see the recipients in the To-field. To actually send you must add the e-mail adress to Smtp.RcptNames.Add('email@example.com').
Even though it is possible to omit this property some e-mail providers might consider your e-mail as being spam and hence trash it or even not receivie it at all.