Subscribe by Email fails to complete send with large subscriber list

As started here - https://premium.wpmudev.org/forums/topic/tried-to-use-subscribe-by-email-plugin-again-today-for-a-weekly-digest-and-it-stalled-after-236-of-12700?replies=8#post-646228

Subscribe by Email completes sends to a small percentage of the total subscriber list when the list is ~7k.

Details:

Using wordpress SMTP plugin
WHM/Cpanel set to allow large volumes of emails
7k subscribers
Notification period is "immediate"