Dropped emails are similar to the receiving server simply deleting the email message.
In those cases, the gift was not delivered to the recipient, even if they look for it in their spam folder. Dropping a message is rare, though it can happen.
Unfortunately, it's unlikely that you'll be able to deliver gifts to that email address. You may want to make sure they have a phone number entered so they can receive gifts by text, or you can try changing their email to a different address.