MaxBulk Mailer settings panel shows an option with a checkbox labeled 'Singly'. That option can be activated/checked or deactivated/unchecked. It is checked by default.
You will find below an explanation of what happens to the delivery of your message depending on whether the 'Singly' option is activated or not.
Singly Activated (Default) - When the 'Singly' option is activated MaxBulk Mailer sends your message separately to each addressee, that is, one fully customized message per recipient with the 'To' header set to the recipient name and address. It is like if you were composing each message specially for each recipient. The level of customization is high since you can use tags to insert recipient data into the message, making it look like the same as if it were sent to that recipient only. In addition this type of delivery allows you to track clicks on links with MLM (the MaxBulk Mailer tracking script), lets you know which recipients have opened your message and allows you to handle unsubscribe requests and bounces with MLM. The only drawback is speed since you will send the message contents as many times as recipients in your list and if your message contains attachments be careful of their size since, again, you will send them as many times as the number of recipients in your list (more info about this here). It is actually recommended not to add attachments to your message when the 'Singly' option is activated, better upload the files you want to attach to your web server and add a link to them in your message. That's much faster and you will avoid the well-known problem of attachments being removed by servers, routers and hardware anti-virus appliances along the way.
Singly Deactivated - When the 'Singly' option is deactivated MaxBulk Mailer sends your message in bulk mode. You probably don't know right now what the bulk mode is but in email reader software like Apple Mail, Entourage or Outlook it is called 'Bcc' which stands for Blind Carbon Copy. Indeed when you send a message using one of those software with several addresses in the Bcc field what actually happens is that the list of addresses is sent first to the server followed by a single copy of the message. As a result the 'To' header will no longer contain each recipient name and address but something like 'Undisclosed recipients'. What happens is that the server is now the responsible for the message dispatching. MaxBulk Mailer lets you change that 'Undisclosed recipients' string to something else but whatever you choose, it will be the same for every recipient in the list. MaxBulk Mailer also lets you set how many recipients to send per message to avoid server errors on big lists. The level of customization is low since you can't use tags to insert recipient data into the message anymore, just the generic tags like dates are allowed. Furthermore this type of delivery doesn't allow you to track clicks on links nor it lets you know which recipients have opened your message. To handle unsubscribes you will have to use a sentence asking the recipient to reply to unsubscribe. The only advantage is again speed, indeed bulk mode is faster, much much faster. The reason is simple, you only send your message once and if the message contains attachments, they will be sent once per group.
So, what is the best delivery method? It is up to you actually. It depends on the level of customizations you want for your message, the need of tracking data and the presence of attachments.