Usually, when ordering invitations either in person or online, you have the option to print the return address on the back flap of the outer envelope as well. I will often hand-calligraph return addresses in addition to the guest addresses, but in general this is what I see when I receive outer envelopes for formal or special events:
(photo from merrybrides.tumblr.com)
However, at times when ordering, you may forget to request this, or it may not be available. Every once in a while someone may bring me completely blank envelopes -they may not have budgeted for the return addresses either on the back flap or the response card envelope, or printing it simply slipped their minds (it happens). Then they need to make a split decision which is either inconsistent with the rest of the suite or not what they planned to do. Whoops!
TIP: Make a mental – or physical – note for yourself to remember the return addresses when ordering your invitations. This way, you can make sure you have it printed on your invitation suite, or budget for having it done in hand calligraphy. Liners in the envelope flaps may affect the outcome of the ink, so use your best judgment or ask a professional for their opinion.
I’m always glad to help!