Some mail servers may not allow too many emails to be sent in a short time. You can spread out sending mail merge and bulk emails using the "Bulk Send Delay" action.

Select multiple emails in the email drafts screen and then select "Bulk Send Delay" from the action menu in the top right.
This will open the Bulk Send Delay screen. You can edit the number of emails to send in each group. You can also choose the time delay between each group in minutes.
Tap on the Send button to start sending the emails after a short delay. In the email drafts screen you will now see the scheduled send time for each email according to your group choices.
If you want to cancel the schedule for sending emails, select emails and then choose Send later in the action menu. Then choose None to clear all of the schedules for the selected emails.
You can also select emails and use the "Bulk Send Delay" action again to reschedule emails.