At this point, your email server should get up and go get you something to drink in appreciation for all the attention it has received in favor of your spouse. It may seem like a lot of work, but user authentication is a fairly complex configuration. It certainly could be a lot easier, but then I would have less to write about.
So where should you go from here? Straight to the documentation. The Postfix documentation is unusually good, and is rife with examples. The documentation is all on-line at the Postfix Web site: http://www.postfix.org/
The Postfix Mailing List archives are another excellent place to read up on and ask questions about Postfix. The wonderful thing about open source projects such as Postfix is that you can ask questions and actually receive answers from the individuals who know best: the developers. The Postfix Mailing List archives and subscription info: http://www.postfix.org/lists.html
A little concerned about sending user accounts, passwords, and sensitive email using unencrypted SMTP? Virtual Private Networks (VPNs) may be an answer, but they carry a large administrative overhead. Perhaps looking at adding SSL/TLS support to Postfix will answer your security concerns. Using SSL/TSL to secure SMTP will help in the same way that SSL/TLS has secured http for the Web. Unfortunately, SSL/TLS is not yet part of the default Postfix distribution. However, there is a well-maintained patch to Postfix that adds SSL/TLS functionality. The Postfix TLS patch: http://www.aet.TU-Cottbus.DE/personen/jaenicke/postfix_tls/