Auotomating e-mails question

Maybe it's a stupid question, but here it is:

I have a list of 1,000 people but MailerLite only lets me put in 500 at this time.

I sent an e-mail and 274 recieved it (because of free plan I think?)

What happens to the rest?

Also - how do I make sure the next time I send an e-mail it goes to the people who did not recieve it?

Thanks