Try to find the dovecot binary in more places.

see #510

doc/integration/imap_and_smtp.rst

@@ -55,6 +55,15 @@ For dovecot 2.0 and older, use the `autocreate
 Operations on the file system
 -----------------------------
 
+.. warning::
+
+   Modoboa needs to access the ``dovecot`` binary to check its
+   version. To find the binary path, we use the ``which`` command
+   first and then try known locations (:file:`/usr/sbin/dovecot` and
+   :file:`/usr/local/sbin/dovecot`). If you installed dovecot in a
+   custom location, please tell us where the binary is by using the
+   ``DOVECOT_LOOKUP_PATH`` setting (see :file:`settings.py`).
+
 Three operation types are considered:
 
 #. Mailbox creation

modoboa/core/commands/templates/settings.py.tpl

@@ -172,6 +172,8 @@ SOUTH_TESTS_MIGRATE = False
 
 #MODOBOA_CUSTOM_LOGO = os.path.join(MEDIA_URL, "custom_logo.png")
 
+#DOVECOT_LOOKUP_PATH = ('/path/to/dovecot', )
+
 AUTHENTICATION_BACKENDS = (
     'modoboa.lib.authbackends.SimpleBackend',
 )

modoboa/extensions/admin/app_settings.py

@@ -1,4 +1,5 @@
 from django import forms
+from django.conf import settings
 from django.utils.translation import ugettext_lazy
 from modoboa.lib.formutils import YesNoField, SeparatorField
 from modoboa.lib.sysutils import exec_cmd
@@ -49,10 +50,13 @@ class AdminParametersForm(parameters.AdminParametersForm):
         hide_fields = False
         dpath = None
         code, output = exec_cmd("which dovecot")
-        known_paths = ("/usr/sbin/dovecot", "/usr/local/sbin/dovecot")
         if not code:
             dpath = output.strip()
         else:
+            known_paths = getattr(
+                settings, "DOVECOT_LOOKUP_PATH",
+                ("/usr/sbin/dovecot", "/usr/local/sbin/dovecot")
+            )
             for fpath in known_paths:
                 if os.path.isfile(fpath) and os.access(fpath, os.X_OK):
                     dpath = fpath