as noted you need to ask sendmail to do what you want (defer delivery).
current versions of sendmail can do this and in fact if you want to be
able to deliver very much* mail sendmail has to be configured to do only
the minimum necessary.
 
*much mail is ~250k messages a day to other machines.
 
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Now, suppose that you are disconnected from the rest of the InterNet, case
in point our provider's router died. Now, sendmail still tries to do the
DNS lookup, but it now times out after 30 seconds. If you have a list with
many recipients, these 30 seconds add up for each timedout DNS lookup.
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 paul