Thank You for all your Reply guys... it really helped me find a solution for this inconvenience. I personally can't afford VPS hosting nor pay extra for an external for a mail server...
Considering Jonathan Solution and William's Comments i ended up developing a small php application based on XML to send different batch to 250 recipients each with a GAP of 65 minutes.
So the way it works, by default it only enables the first batch link to be clicked and send the newsletters to the first batch of users and recording the exact time this was sent in a XML file.
Then using the XML file info the next link registers that the batch before it was sent and starts a count down of 65 minutes with the time on the XML as reference.
So a Script will not be running for hours and the browser could be safely close since all the info required is in the XML file.
This may sound Simple but is a complex and efficient app that dynamically adapts to growth (new subscribers) as it queries the master Table on a database using the sql LIMIT clause to make the different batches. So it doesn't required maintenance.
If anyone is interested on the source code feel free to contact me @ [email protected]