Best way to bulk email an attachment? I need to take a file, and email it to a large group of people. Naturally, I'll be using a custom plugin Controller to trigger the email blast.

I'm already aware of this similar thread, but I don't believe it will solve my problem. If I remember correctly, Craft's native email model isn't capable of sending to more than one person at a time. More importantly, I don't want our server to be labeled as a spammer!

Would it make sense to run this process through a Task? Each individual email could be sent out as a separate step in the task.

What is the best way to bulk email an attachment?
There's nothing that says you have to go through Craft's EmailModel/EmailService for this.

Craft ultimately uses the PHPMailer library, so you can instantiate it directly and use whatever settings you need (or use a completely different library).

I assume you don't want to cram everyone into to "To Address", so is the plan to set the "To Address" to yourself and BCC the recipient list?

Depending on the number of emails you need to send, I'd probably just have your plugin read from the file and insert into a database table.  Then you could setup a cron job to hit your plugin's controller action to send out (one at a time) X emails over Y time period and delete those from the table until its empty.

As far as not getting flagged as a SMTP relay, in addition to throttling email rates, there are definitely other things you can do to reduce your chances.