Feature request: Load balancing adapters

[nickjj]

Use case

You have a mailing list with 500,000 people and mailing back-ends such as Amazon SES and SendGrid enforce limits such as 10 mails per second or 50 mails per second.

This increases the total send time for a "campaign" and is most definitely the biggest bottleneck when it comes to sending out a lot of e-mails in bulk.

Feature

It would be really nice if you could configure something to load balance multiple adapters that are configured. The first implementation could be a round robin balancer which simply alternates sending e-mails out to each adapter.

Down the line (or now if it's not difficult?) it would be interesting if you could supply your limit to the adapter in the form of e-mails per second and then introduce an algorithm that determines the fastest way to deliver all of your e-mails.