At about 3:30pm PST, we became aware of email delivery issues. At present, they look to be caused by an error at SendGrid.com, whom we use to send mail.
We are working on resolving this situation, and will update here when more info is available.
All transactions are processing normally, but email receipts (and other emails, such as password resets) are not being delivered.
UPDATE 5:23pm PST: Emails look to be sending, but we are still working to ensure all emails that were attempted during the impacted window actually go out. SendGrid.com's status page still shows "Degraded performance": https://status.sendgrid.com/