We take steps on our end to try to ensure Substack emails reach the reader's main inbox but over time the reputation that your newsletter builds up with readers it the most important input.
Asking your readers to drag the emails back to the Inbox helps. Also, you can encourage them to write to you at your @substack.com address, which helps inbox providers help recognize your emails.
Tips for readers on how to move emails:
Move the email newsletter to the Primary inbox by selecting the email and dragging it to the Primary tab.
For Apple Mail
Tap on the sender name and select "Add to VIP".
Follow the steps here to add @substack.com as a contact.