This value is only used during GSSAPI authentication (Kerberos or NTLM). If it's a null reference, MailBee will construct it as "SMTP" + "/" + ServerName. Or, if it's set to a non-empty string, its value will be used as-is. If it's an empty string, GSSAPI authentication will perform as NTLM (not as Kerberos).
You may need to set this property if you want to authenticate with Kerberos and your SMTP server has SPN different from "SMTP" + "/" + ServerName. For instance, IIS SMTP may use "SmtpSvc" + "/" + ServerName.