Retry delay or queue

[MaximilianKohler]

This would fix a few different issues: #1717 (comment) and #1992

Currently, I have to go through an extremely tedious process every time I send a campaign: #1629 (comment)

I never get errors when resending to the subscribers that first errored, so it's clear that a delay is needed.

Describe the solution you'd like

Under SMTP settings, add an option for a retry delay (Ideally a progressive one like the Amazon rep described), or automatically send retries to the back of the queue.