Emails in a Multisite Instalation

Hi,

I have a multisite instalation and I have doubts on emails. My clients will create their sites by using the network and they want a business email. They can host their emais in any company and each company provides its own MX registers to configure the email.

How does it works in a network? Do I have to manually configure the email for each site depending on the email server? I'm very confused.

Thanks.