In order to take care of a large company’s email needs, the outsourcing company you select needs to be at the top of its game, with large servers and comprehensive customer support in order to really take the lead in this market.
A company you select should have/include:
A General Level of Competence
This may seem obvious, but the fact is, many people will not seek to validate claims of prior competence on the part of an outsourcing company. When such validation is undertaken, you may find that the company is too new to have testimonials, or that those testimonials are from unknown companies in distant parts of the world—not a great start to proving competence.
An email service provider should be able to provide proof of experience, references, and contact numbers so that you can ask any questions of those references. Simple quotes on a testimonials page of a website should not suffice. Additionally, investigate documentation for the provider's procedures and policies in any eventuality. They should have policies in place for crashes, data restoration, backups, and security breaches.
Reasonable Costs
It is reasonable to see an email provider offering their services for a monthly, per-mailbox fee. This is the industry standard and should be expected. What you need to know is how many additional services are included in the base, per-mailbox price. It is also reasonable to see additional fees for additional services. The industry often charges slightly more for features such as additional storage, secure socket layer (a type of encryption) and possibly for virus and spam filters, though increasingly virus and spam protection is falling under basic standard service agreements.
You should also calculate the cost of bandwidth usage before committing to a service provider. This is a cost that will increase the average monthly per-mailbox cost. This is out of the provider's control, but it still needs to be calculated.
You will also need to calculate the cost and effectiveness of the company's SLA or Service Level Agreement. This is an agreement between you and the contractor that will need to cover the availability of system access or "uptime" and general guidelines on availability and performance. This agreement should have reasonable components and be part of the base price, as it should be the most fundamental selling point for the company, so far as customer service is concerned.