feature: compose: allow not storing the sent emails

I'm using lieer/gmi to send email, and it has the builtin assumption
that no new messages will be added to the maildir. See gauteh/lieer#54

dodo/compose.py

@@ -480,12 +480,18 @@ def run(self) -> None:
             self.panel.status = f'<i style="color:{settings.theme["fg_bad"]}">timed out</i>'
 
 def _store_sent_email(eml: email.message.EmailMessage, account):
-    # save to sent folder
-    m = mailbox.MaildirMessage(str(eml))
-    m.set_flags('S')
+    # Pick the right directory
     if isinstance(settings.sent_dir, dict):
         sent_dir = settings.sent_dir[account]
     else:
         sent_dir = settings.sent_dir
+
+    # None means we should discard the email, presumably because it's already
+    # handled by whatever mechanism sends it in the first place
+    if sent_dir is None:
+        return
+
+    m = mailbox.MaildirMessage(str(eml))
+    m.set_flags('S')
     key = mailbox.Maildir(sent_dir).add(m)
     # print(f'add: {key}')

dodo/settings.py

@@ -53,6 +53,10 @@
 :func:`~dodo.settings.sync_mail_command`. This setting can be given either
 as a string to use one global sent directory, or as a dictionary mapping
 account names in :func:`~dodo.settings.smtp_accounts` to their own sent dirs.
+
+A value of None, either standalone or as one of the dict value, can be used to
+indicate the email should be discarded. This can be useful if the sendmail
+command already has a mechanism for that feature.
 """
 
 editor_command = "xterm -e vim '{file}'"