Thanks for reaching out. What you'll want to do is configure SMTP Accounts for the user. These can also be used in conjunction with Message Retrieval so users can send and receive emails for multiple accounts from one account. I believe the Online Help documents should give you all the info you need to know, but of course, if you have any other questions, don't hesitate to ask!
I'll also get the Composing Email Messages
doc updated so it links to this information, in order to avoid confusion in the future.