diff --git a/extra/modules/bodiffusion.ptl b/extra/modules/bodiffusion.ptl
index 777756e..51c5a7b 100644
--- a/extra/modules/bodiffusion.ptl
+++ b/extra/modules/bodiffusion.ptl
@@ -558,9 +558,14 @@ class DiffusionDirectory(Directory):
         self.html_top(_('Import a List of Participants'))
         '<h2>%s</h2>' % _('Importing a List of Participants')
         '<p>'
+        _('The file should be in the CSV file format. Using your spreadsheet '\
+          'program (Calc, Excel...), click "Save as" and select the CSV format.')
+        '</p>'
+        '<p>'
         _('The file should have email addresses in the first column, and, '\
           'optionnaly, names in the second column.')
         '</p>'
+        get_session().display_message()
         form.render()
 
     def import_submit(self, form):
@@ -611,8 +616,25 @@ class DiffusionDirectory(Directory):
 
         tmpfile = tempfile.NamedTemporaryFile()
         fp = form.get_widget('file').parse().fp
+        dialect = None
         while True:
             s = fp.read(1024*1024)
+            if s and not dialect:
+                try:
+                    dialect = csv.Sniffer().sniff(s, delimiters=',; \t')
+                except csv.Error:
+                    # perhaps this is just a list of emails, and it raised
+                    # "Could not determine delimiter", so we check the first
+                    # line
+                    if '\n' in s:
+                        first_line = s[:s.index('\n')]
+                    else:
+                        first_line = s
+                    if first_line.count('@') != 1:
+                        tmpfile.close()
+                        get_session().message = ('error', _('Failed to use the file, please check its format.'))
+                        return redirect('import')
+                    dialect = 'excel'
             tmpfile.write(s)
             if not s:
                 break
@@ -742,8 +764,13 @@ class DiffusionDirectory(Directory):
         self.html_top(_('Import a List of Participants to Disable'))
         '<h2>%s</h2>' % _('Importing a List of Participants to Disable')
         '<p>'
+        _('The file should be in the CSV file format. Using your spreadsheet '\
+          'program (Calc, Excel...), click "Save as" and select the CSV format.')
+        '</p>'
+        '<p>'
         _('The file should consist of email addresses, one per line.')
         '</p>'
+        get_session().display_message()
         form.render()
 
     def import_disabled_submit(self, form):
@@ -792,8 +819,26 @@ class DiffusionDirectory(Directory):
 
         tmpfile = tempfile.NamedTemporaryFile()
         fp = form.get_widget('file').parse().fp
+        dialect = None
         while True:
             s = fp.read(1024*1024)
+            if s and not dialect:
+                try:
+                    dialect = csv.Sniffer().sniff(s, delimiters=',; \t')
+                except csv.Error:
+                    # perhaps this is just a list of emails, and it raised
+                    # "Could not determine delimiter", so we check the first
+                    # line
+                    if '\n' in s:
+                        first_line = s[:s.index('\n')]
+                    else:
+                        first_line = s
+                    if first_line.count('@') != 1:
+                        get_session().message = ('error', _('Failed to use the file, please check its format.'))
+                        tmpfile.close()
+                        return redirect('import-disabled')
+                    dialect = 'excel'
+
             tmpfile.write(s)
             if not s:
                 break