Sendible says it is “the complete messaging platform”. What Sendible does is 

Connect with all your email, mobile and social network contacts even when you’re away from the computer

•  Schedule email, sms and social network messages ahead of time
•  Access all your email and social network contacts from one place
•  Remind yourself and others of upcoming tasks and events
•  Post status updates to your blogging and social network accounts

Sendible has a range of free and paid accounts to select from.