External services and consultants

What sources do you provide in-house? What do you outsource and why? Office 365? Web services? Email? Phone services? Other things?
What are the pros and cons of hosting these services internally versus getting somebody else to do them? What factors do you consider when outsourcing a service?

How do you deal with people providing external IT services to your organization? How do you interact with consultants and other external service providers effectively?