After authenticating your domain, there is a throttling process that happens for your first newsletter send to help warm your domain. This introduces your domain and typical sending volume to inbox providers. For maximum effectiveness, send your first newsletter to all subscribers.
Do not send any content that is very time sensitive because it is a slow process and you will not be able to stop sending after it starts. You will see a banner atop the newsletter detail page describing about how long it will take. And it's only the first newsletter send that needs to go through this process, subsequent ones will be sent all at once.
Essentially, it starts by sending 20 emails and then sends double the previous amount every 4 hours. We also sort your subscribers to start with your most engaged first. If you're coming from Mailchimp, please make sure to connect our integration at the bottom of our Settings page as some of that data will improve our ability to target more engaged subscribers first.
This warming process is a slow start, yes, but a very important step to help with your deliverability and we handle it automatically. Again, this is only for your first newsletter send after authenticating your domain.