how to send massive group email?

Hi Everyone

Can someone tell me how to send a group email to a high number of people please? I have edited an email (15k lightweight) to send to a group of people but the email server says it did not recognised the recipients...😕 Have I done something wrong? It's a promotional email from my company so I need to keep contact with my customers. I prefer having advice rather than doing something wrong which I couldn't undo later...

Thank you everyone for your help...

Fred