If you’re not receiving the opt-in confirmation email or the final welcome email, please check the following:

• Is an error showing after submitting the form?
• Is there an error message in the debug log, on the MC4WP > Other page in your WP Admin?
• Do you receive the confirmation email if you attempt to subscribe with another (new) email address? Mailchimp will often stop sending confirmation emails to the same email address, so trying with another address often fixes the issue.

If you use the same email to test one or multiple forms in a short amount of time Mailchimp might temporarily block your email from signing up to any Mailchimp list.
Trying another (fresh) email address often solves this issue.

If the form shows a success message and there are no error messages in the debug log then it means that the plugin successfully sent the sign-up attempt to Mailchimp. From there, it is up to Mailchimp to send & deliver the confirmation email.