Make ActionMailer non-blocking, like ARMailer
Calls to ActionMailer currently block the client until the mail is sent. This is notably sub-optimal (and can lead to timeouts) when sending batches of emails from one action.
Integrating something like the ActiveRecordMailer, or another system for queuing messages, could be a real improvement for this situation.
I've configured sendmail to use its queue in my pre-ruby days... This is admittedly far from as easy as if it were built in, but it can be made to work with most languages/frameworks that doesn't provide this built in yet.