Simplest way to allow users to email their followers.

I'm researching building a platform for users to easily gather email subscribers and contact them, for example to allow Indiehackers to gather subscribers for product updates. It seems my options are:

1. Integrate with providers like Mailchimp to collect subscribers and allow for easy sending of emails to subscribers
2. Collect subscribers in my own DB and build my own mailing system to allow owner to email subscribers.
3. Collect subscribers in my own DB and just allow owner to download as CSV and find their own way to email subscribers.

I want to avoid #1 was it really dilutes my value, at that point you might as well just use Mailchimp and not me even though I'll be adding nice features on top.

#3 would be the easiest, as it skips the emailing feature all together, but being able to contact your subscribers would be a very valuable feature.

#2 is the ideal solution, however afaik sending emails customer-to-customer is a tricky feature. First you have to handle allowing users to build templates/content that are compatible with email providers. You also have to prevent spam between users. I'm thinking if I went this route I would just allow users to send simple text/markdown emails to their subscribers, this would avoid most of the compatibility/responsiveness issues. I'd also use an email service like Sendgrid, but I'd still have to find a way to prevent my service from being blacklisted or sent to spam if my customers subscribers are bouncing.

How would you build a service like this?