The inner envelopes that come with your invites CAN BE MAILED. However you can decide to add a bit of formality to your ensemble by adding OUTER ENVELOPES.
Outer envelopes are envelopes that serves mailing reasons, they were invented in a time when invites were hand delivered and were going to go through any kind of weather. In a formal invite the family name is printed on the Outer envelope, and inside of it there are different Inner envelopes, each addressed to one or more family members. Please note that these are NOT NECESSARY.