+// FEATURE: batching retrys by host (retry multiple in one session, keep retry intervals on a host basis not a message basis)
+// FEATURE: first two attempts should be close together (rec'd by 2821)
+/*
+ // FEATURE: RFC2822, section 4.5.4.1: retry strategies
+ // per-command, per-attempt timeouts
+ Experience suggests that failures are typically transient (the target
+ system or its connection has crashed), favoring a policy of two
+ connection attempts in the first hour the message is in the queue,
+ and then backing off to one every two or three hours.
+
+ The SMTP client can shorten the queuing delay in cooperation with the
+ SMTP server. For example, if mail is received from a particular
+ address, it is likely that mail queued for that host can now be sent.
+ Application of this principle may, in many cases, eliminate the
+ requirement for an explicit "send queues now" function such as ETRN
+ [9].
+
+ An SMTP client may have a large queue of messages for each
+ unavailable destination host. If all of these messages were retried
+ in every retry cycle, there would be excessive Internet overhead and
+ the sending system would be blocked for a long period. Note that an
+ SMTP client can generally determine that a delivery attempt has
+ failed only after a timeout of several minutes and even a one-minute
+ timeout per connection will result in a very large delay if retries
+ are repeated for dozens, or even hundreds, of queued messages to the
+ same host.
+
+ When a mail message is to be delivered to multiple recipients, and
+ the SMTP server to which a copy of the message is to be sent is the
+ same for multiple recipients, then only one copy of the message
+ SHOULD be transmitted. That is, the SMTP client SHOULD use the
+ command sequence: MAIL, RCPT, RCPT,... RCPT, DATA instead of the
+ sequence: MAIL, RCPT, DATA, ..., MAIL, RCPT, DATA. However, if there
+ are very many addresses, a limit on the number of RCPT commands per
+ MAIL command MAY be imposed. Implementation of this efficiency
+ feature is strongly encouraged.
+ */