If Interspire Email Marketer says that it has sent to less then the amount that it should have but also states that it has completed sending then some emails have been rejected from your mail server for some reason. Interspire Email Marketer will simply receive a 'Yes I accepted the email' or 'No I did not accept' message from the mail server. This means that there is no way to determine the exact reason why the message was rejected.
This could be because your host has a per hour sending limit in place to slow down all sending on the server. If this could be the case you should contact your host to see if they have a limit and what it is and then set this limit in the Interspire Email Marketer settings page so that you do not send faster then this limit and risk your emails being blocked by the server.
If this is not the case and your hosts do not have any limitations in place you should ask them to see if the mail server was down for any time whilst your send was in place. If they say that it was not down then you can request the server logs for that time frame to see if there was any errors recorded that may inform you as to what has happened.

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