Using morbo, debug return the lines below:

pazt@mojo:~/theo$ morbo myapp.pl
Server available at http://127.0.0.1:3000
[Sun Sep 11 02:12:43 2016] [debug] POST "/sign_up"
[Sun Sep 11 02:12:43 2016] [debug] Routing to a callback
[Sun Sep 11 02:12:43 2016] [debug] Your secret passphrase needs to be 
+changed
[Sun Sep 11 02:12:43 2016] [debug] 302 Found (0.041378s, 24.167/s)
[Sun Sep 11 02:12:43 2016] [debug] GET "/"
[Sun Sep 11 02:12:43 2016] [debug] Routing to a callback
[Sun Sep 11 02:12:43 2016] [debug] Rendering template "index.html.ep" 
+from DATA                                                            
+                           section
[Sun Sep 11 02:12:43 2016] [debug] 200 OK (0.005431s, 184.128/s)
[download]

postfix is installed on my server to send emails.

I think you want to look at the Minion log, as that is the one actually trying to send the mail and marking the mail job as "done".