If you are attempting to send a campaign from a public domain (ie. Gmail, Yahoo, Hotmail, etc) you will likely see an error message like this:
Why shouldn't I send from a public domain?
Most email providers will block or otherwise filter emails coming from a private domain. Why do they do this?
There are a few reasons: 1) they want to block against spammers trying to co-opt the good reputation of a public domain to get their emails into the inbox and 2) providers want to know more about WHO is sending this email to make sure it is not dangerous for their customers. Sending from an owned (private) domain let's providers see WHO is actually sending this email and allows them better information to determine whether or not the email should make it to the inbox.
Finally, it is key to have proper authentication set up for your sending domain (see more here) which cannot be achieved using a public domain.