Topic: Built-in Newsletter functionality

I've searched around for this one for some time (Ruby Toolbox, Google, SO) and didn't find a satisfactory answer, so I'm hoping someone could point me to a good gem or let me know a best practice or something. I think my problem is fairly common. I want to build easy newsletter function into my app, so that admins can send messages to all users (or particular groups of users) that are sent with the app's basic letterhead in the same way the ActionMailer emails that are triggered by events are. I'm using Sendgrid, and the basic event-triggered emails are working just great.

I'm open to anything, really. I'd go ahead and just build a Newsletters controller and have the create action in my view (after I enter the content) just ship it off, but a) I'm worried about any mis-formatting I might do (if I don't close a tag or something), b) I'd like the option to more easily format in plaintext/html, and c) I have a lot of models already, and I don't want to start overcomplicating things for myself if there's a good option out there. Any quick suggestions about gems that are good for this or the best ways to roll it out by hand? Articles to look at, etc? Thanks.