postfix-relay | Postfix SMTP relay docker image | Continuous Deployment library

I've already answered my own question and may later update this question to reflect on my starting point/the steps I took to get to my solution, but figured I would ask a question that I started with and the result that took me an unreasonable many hours of research, and trial and error, to get to. Please make any edits, or propose your own postfix/main.cf solution as I still have much to learn.

Introduction to Problem

So I self-host a few things on my server at "example.com" and set up Postfix as my mail transfer agent (MTA). On my mail server, I have a virtual_alias setup to receive emails for particular "email_users@example.com" to my username on the server. My Alma mater has email forwarding enabled so that emails to "student@college.edu", or "alum@alum.college.edu" are forwarded to "email_users@example.com" and received in my user inbox. Essentially all emails (to my .edu or my .com) go to /home/user/Maildir/new.

When writing emails using MUTT (my preferred MUA), I will occasionally change my email "FROM" field to be "student@college.edu", "alumnus@alum.college.edu", or by default reply with whatever reply-to field is enabled. The desired behavior for my mail server outbound to other servers is as follows:

1. On emails with "FROM: *@example.com" - route the email via SMTP through local Postfix MTA directly to the internet. (* representing wildchar)
2. On emails with "FROM: student@college.edu", route the email via SMTP through local Postfix MTA and relay it to another SMTP server WITH authentication so that the other server delivers without any soft-fail or bounces (in this case the college SMTP server).

To be clear, this is a question of configuration of Postfix when the user would like to send mail from: local Postfix MTA -> external SMTP server -> recipient via internet.

These questions/how-tos have generally omitted a clear answer, are not asking the same thing and require a better asking title, or are how-tos that only begin to answer the beginning of this setup:

• Postfix: Relay mails from specific domain
• https://serverfault.com/questions/257637/postfix-to-relay-mails-to-other-smtp-for-particular-domain
• https://www.howtoforge.com/postfix_relaying_through_another_mailserver#-configure-postfix-for-relaying

Of course the full documentation is helpful, but quite verbose and hard to figure out in a timely manner if you are new to Postfix. For instance, you may expect to find this under SMTP Relay/Access Control, but the main aspect I was missing was under general configuration in SASL Auth.