When you receive customer questions that they don't receive your emails, they might have entered the wrong email address during checkout.
In the portal you can find a customer based on their email address, subscription ID (if known) or part of their name.
Check email details
Under Subscription details the email address is shown. In the example below, it shows that the customer entered @gmaill.com (double L) and therefore didn't receive any emails. Use the Edit button to, for instance, change this to gmail.
Re-send email notification
If the customer contacted your customer support because they didn't receive an email, you can resend them via the subscription profile under Email notifications.