Defines the e-mail address of recipient(s). These are seperated by ; and an e-mail adress can either look like firstname.lastname@example.org or My name <email@example.com>. Note that HdrCc is not used to actually send the e-mail, HdrCc is used for e-mail clients to visually see the recipients in the CC-field. To actually send you must add the e-mail adress to Smtp.RcptNames.Add('firstname.lastname@example.org').
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. Note that there is no such thing as HdrBCC since the whole purpose of BCC is to not see the recipient and it's therefore never included in the e-mail.