The mailing will automaticly start from the addresses that have not recieved it the first go round, on creating a new mailing there is not a way to target the users that didn't receive a previous mailing.
If the one mailing is sending, but the other isn't, i would suspect that there is a bad email address in the uses table.
ass for breaking up the subscribers, you can create mailing audiences. There are a number of different ways you can specify the audiences, you could do it by user state, create an audience for each state, you could do it by last name alphabetically, for example:
UserLastName Begins With "A"
and create a separate mailing group for each letter of the alphabet, there are a myriad of ways to set up the mailing audience.
