Many Internet hosts are not in a position -- whether technically,
operationally, or administratively-- to offer email service. If an
SMTP client (sender) attempts to open a mail connection to a system
that does not have an SMTP server, the connection attempt will time
out.
How about just making it "will be rejected or time out."
SMTP requires that timeouts result in the client queuing the
"that timeouts" -> "that such failures"
message and retrying it for an extended period. That behavior will
result in wasted resources and long delays in getting an error
message back to its originator.
It's shorter, says what needs to be said.
R's,
John