Good morning Sean. You mention, "we will begin blocking unsecured outgoing SMTP connections from Opalstack web servers by blocking outgoing port 25 in our network firewalls."
What about mutt?
When Webfaction made a similar change in 2016, a certain seanf replied in response to query, "If you have configured mutt to use an external SMTP server, then you will need to check your mutt configuration to ensure that it is connecting to that external server via 465 or 587. If you have not configured mutt to use an external SMTP server, then it is using your server's local sendmail, and you don't need to change anything."
Does that still apply?
While on the subject, in 2017 Webfaction experimented with a further change: "By default, mail sent through sendmail is from a email@example.com address, where user is your username and host is the name of the server. Our mail system will not allow messages to be sent from such addresses, so be sure to set a valid sender address in your application."
They then reverted the change (I think because it was disrupting some customers' wordpress sites). Is that still a potential issue?