So I exported all my subscribers from mailchimp and imported them into tiny letter, when I tried to use their newsletter service to inform my subscribers of a facebook group that I have created for them, Tiny Letter's system flagged my newsletter, I got in touch with them and here's what they told me:
We have nothing personal against you or your content, but as a permission-based email service, we can't support prospecting or introductory campaigns of any kind because of the complaints they generate.
We have to ask that you seek a new vendor for your email marketing needs.
We appreciate your understanding in this matter."
I should be the one asking what proof do they have that my contacts didn't subscribe to my newsletter. Do they want me to send them all the email notifications that I've got from mailchimp when my contacts subscribed? That's just crazy. Stay away from them. They're probably stealing people's data themselves and then accusing people of sending unauthorized emails.