as described in my post here Install Discourse on Amazon Web Services (AWS) with Lightsail, SES, and Route 53 - #4 by wal I was able to send test emails prior to running the full discourse-setup script using this method described here with telnet
notably, I used port 587 instead of 25 shown there, and you need to be sure to base64 encode your SMTP username and password.
Also linked in that post is a script that AWS provides which shows how to do the same with the openssl command (link here; Creating configuration sets in SES - Amazon Simple Email Service )
if you are not using AWS SES for your email server you may have to tweak the script a little but in general I think one or both of these should be helpful for you to verify email before the installation (this is what I did)
while I agree that there should ideally be a way to test the email prior to installation, I think it may actually be preferable to have it shipped as a separate stand-alone script of some sort with Discourse so that you can run it without having to go through the entire setup process, which can take ~10min