You usually want to send the e-mail in order, with a sleep in between to not overload the e-mail server, and stop at any error, so that you can fix the error and then continue from where the error was.

An error could be a malformed e-mail address, or a timeout from the e-mail server.

What you do not want is to send 10000 e-mail, then have an error like "Error sending emails", then re-run a few time, only to have some people receive 5 e-mail, and some people receive 0 e-mail.