Outlook & exchange slow email sending

My Microsoft outlook 365 client is connected to my 3rd party exchange server but when I send bulk emails it only loads one email at a time e.g if I send an email to 100 people it will load one email into outbox, send it out, wait 2/3 seconds then loads the next email and repeats. This problem started when I changed my computer prior to this it would load the emails in the outbox and send them out simultaneously as it's loading them up so what used to take 1 min to send 100 emails no takes around 10 mins.

The only think that's was touched when making the change of computers is the ost files that could have an effect on this but don't take my word for it

Please give me an overview of what you would be looking to do do and how long the project will take as I am very short on time

I have not nit put a project price as I'm not sure what the problem is and I'm happy to wait for proposals with a price. I am happy to accept a proposal right away.