What is email deliverability in Salesforce?

Email deliverability is the likelihood of a company’s or individual’s email reaching its intended recipient. Just keep in mind the below points when setting email deliverability:
No access: Prevents all outbound emails to and from users.
System email only: Allows only automatically generated emails, such as new user and password reset emails.
All email: Allows all types of outbound email. The default for new, non-sandbox organizations.