“…In some cases, the domain name(s) transferred to, or determined by, an SMTP client will identify the final destination(s) of the mail message. In other cases, common with SMTP clients associated with implementations of the POP [3,26] or IMAP [6] protocols, or when the SMTP client is inside an isolated transport service environment, the domain name determined will identify an intermediate destination through which all mail messages are to be relayed. SMTP clients that transfer all traffic, regardless of the target domain names associated with the individual messages, or that do not maintain queues for retrying message transmissions that initially cannot be completed, may otherwise conform to this specification but are not considered fully-capable.…”