When sending out a newsletter using the Admin>Newsletter function the advanced settings area allows for selecting Synchronous or Asynchronous send.

Synchronous is one after another, so you send one email when that process is done it sends the next one. Asynchronous starts sending an email and it doesn't care if the first one is done and will start the second, and this goes on until all emails are sent.
 
The Synchronous method is usually preferred but both will work. If you have a large number of mails to send on a regular basis we recommend a service such as Constant Contact.