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[mms] Fix aborting a pipelined DATA command if only a subset of recipients were rejected (Bug #12659).

See: http://www.imc.org/ietf-smtp/mail-archive/msg02687.html

The RFC 2920 text is a bit vague.  Servers must issue a 554 if ALL
recipients are rejected.  This might not happen until after the DATA
command is sent though (which is why we were sending empty DATA data).
Unfortunately, *partial* rejection of recipients will NOT abort the DATA
command. Since we don't pre-verify mail recipients, we want to catch the
case if ANY recipients are bad, since the user generally is expecting
that all e-mail addresses provided will have the message sent to.  This
means we can't pipeline DATA.
Released Horde_Smtp-1.2.2
Development mode for Horde_Smtp-1.2.3