Mass Email Question

I have been working on a mass email program and something is happening that i am not sure is normal or if there is a problem.

If a mass email is created with say 5 address the email is processed immediately and sent by server.

If more than 5 addresses are attached it seems to take groups of 5 addresses at a time and sends them.

Is this normal or do i have a problem.

I noticed this because the addesses i sent to would have the first 5 show up fast then the others sometimes not for 20 minutes or so.

Any help is appreciated.

Jody