This means that you can't configure two different SMTP servers, which is a reasonable thing to want to do in the brave new world of mail failover.
It also means you can't configure two Mailgun accounts or two SES accounts. Although this might seem a little silly, we've had more service disruptions because of policy issues / administrative error (where a particular account was disabled) than actual downtime, so maybe it's not completely ridiculous.
Realign mailers so they can take configuration directly in an explicit way. A later change will add new configuration to take advantage of this and let us move away from having ~10 global options for this stuff eventually.
(This also makes writing third-party mailers easier.)