Thank you for the followup @twisted1919@sami1255 - this is easy to test, can you go to the delivery stats and see how much success vs error/giveup ?
Also, i assume this is a regular campaign not a autoresponder, right?
Do you also have autoresponders in the system? if you do, what is the number of campaigns at once from Backend > Settings > Cron ? If it's 5/10 can you go up, like 50 ?