From a digital marketing standpoint, outbound marketing is viewed as "old and interruptive" ways of fetching prospects and customers.
The outbound marketing term is often used as opposed to inbound marketing and considered as less effective and more intrusive.
"Traditional" outbound marketing techniques are offline marketing techniques - cold calling, direct mail, trade shows, etc. - but also online techniques like display or email via third lists. They are techniques where marketers push their messages out and where messages are intrusive.
For more details about inbound marketing versus outbound marketing, see inbound marketing definition.