I need help on Mass Mailing on Odoo 10 online SaaS. I created a mass mailing and scheduled it to send to a group of recipients. The mass mailing was successfully sent.

However, I created another mass mailing and scheduled it to send to the same group of recipients-- this time it won't send. There is an error appearing on top of the mass mailing: "120 emails could not be sent". I clicked on Retry button and still same issue.

I tried deleting the contacts and reimport them then create another mass mailing and use the same as recipients, then the mass mailing is sent successfully again.

What could be the reason I am not able to send another mass mailing to the same recipients?