Due to the complex nature of email delivery, there are scenarios where some people on your mailing lists may not receive the content (drill or practice plan) you sent to them. Delivery can be affected by user-created filters, spam filtering, corporate firewalls, security policies, etc. Please be sure to have the user double-check their spam / junk mail folders.
If your recipient is still not receiving content you send them, please contact support and include the recipient(s) not receiving the email. Support will be able to determine if the message is reaching the intended mail server, but will not be able to determine if the message actually reaches the recipient's inbox.
Note: email sending is currently a Premium feature