move msmtp configuration out of tutorial, into an obscure section

The vast majority of our users will *not* use msmtp and might be
confused by this, especially in the tutorial. Bury it a little deeper
in the how-to section, which I'll add to in a minute.

howto/submission.md

@@ -62,24 +62,6 @@ says that Access Now! has some docs we could start from. vegas agrees
 that this documentation and support will be covered by teams outside
 of TPA.
 
-## msmtp configuration
-
-This is how dgoulet configured his [msmtp client](https://marlam.de/msmtp/):
-
-    # Defaults for all accounts.
-    defaults
-    auth on
-    protocol smtp
-    tls on
-    port 587
-
-    # Account: dgoulet@torproject.org
-    account torproject
-    host submission.torproject.org
-    from dgoulet@torproject.org
-    user dgoulet
-    passwordeval pass mail/dgoulet@torproject.org
-
 # How-to
 
 <!-- more in-depth procedure that may require interpretation -->
@@ -103,6 +85,40 @@ This document describes the implementation of a **MSA**, although the
 service will most likely also include a **MTA** functionality in that
 it will actually deliver emails to targets.
 
+## More obscure clients configuration
+
+This section regroups email client configurations that might be a
+little more exotic than commonly used software. The rule of thumb here
+is that if there's a GUI to configure things, then it's not
+obscure.
+
+Also, if you know what an MTA is and are passionate about standards,
+you're in the obscure category, and are welcomed to this dark corner
+of the internet.
+
+### msmtp configuration
+
+"[msmtp](https://marlam.de/msmtp/) is an SMTP client" which "transmits a mail to an SMTP
+server which takes care of further delivery". It is particularly
+interesting because it supports SOCKS proxies, so you can use it to
+send email over Tor.
+
+This is how dgoulet configured his client:
+
+    # Defaults for all accounts.
+    defaults
+    auth on
+    protocol smtp
+    tls on
+    port 587
+
+    # Account: dgoulet@torproject.org
+    account torproject
+    host submission.torproject.org
+    from dgoulet@torproject.org
+    user dgoulet
+    passwordeval pass mail/dgoulet@torproject.org
+
 ## Pager playbook
 
 No pager playbook has been built yet. See the [Monitoring and