diff --git app/controllers/mail_handler_controller.rb app/controllers/mail_handler_controller.rb
index 1942ab1..9258cc2 100644
--- app/controllers/mail_handler_controller.rb
+++ app/controllers/mail_handler_controller.rb
@@ -26,10 +26,11 @@ class MailHandlerController < ActionController::Base
   def index
     options = params.dup
     email = options.delete(:email)
-    if MailHandler.safe_receive(email, options)
+    error = []
+    if MailHandler.safe_receive(email, options, error)
       head :created
     else
-      head :unprocessable_entity
+      render :plain => error.join('\n'), :status => :unprocessable_entity
     end
   end
 
diff --git app/models/mail_handler.rb app/models/mail_handler.rb
index d11b6f4..eb49731 100755
--- app/models/mail_handler.rb
+++ app/models/mail_handler.rb
@@ -24,7 +24,7 @@ class MailHandler < ActionMailer::Base
 
   attr_reader :email, :user, :handler_options
 
-  def self.receive(raw_mail, options={})
+  def self.receive(raw_mail, options={}, error=[])
     options = options.deep_dup
 
     options[:issue] ||= {}
@@ -46,7 +46,7 @@ class MailHandler < ActionMailer::Base
     ActiveSupport::Notifications.instrument("receive.action_mailer") do |payload|
       mail = Mail.new(raw_mail)
       set_payload_for_mail(payload, mail)
-      new.receive(mail, options)
+      new.receive(mail, options, error)
     end
   end
 
@@ -86,13 +86,14 @@ class MailHandler < ActionMailer::Base
 
   # Processes incoming emails
   # Returns the created object (eg. an issue, a message) or false
-  def receive(email, options={})
+  def receive(email, options={}, error=[])
     @email = email
     @handler_options = options
     sender_email = email.from.to_a.first.to_s.strip
     # Ignore emails received from the application emission address to avoid hell cycles
     emission_address = Setting.mail_from.to_s.gsub(/(?:.*<|>.*|\(.*\))/, '').strip
     if sender_email.casecmp(emission_address) == 0
+      error << "Ignoring email from Redmine emission address [#{sender_email}]."
       if logger
         logger.info  "MailHandler: ignoring email from Redmine emission address [#{sender_email}]"
       end
@@ -104,6 +105,7 @@ class MailHandler < ActionMailer::Base
       if value
         value = value.to_s.downcase
         if (ignored_value.is_a?(Regexp) && value.match(ignored_value)) || value == ignored_value
+          error << "Ignoring email with #{key}:#{value} header."
           if logger
             logger.info "MailHandler: ignoring email with #{key}:#{value} header"
           end
@@ -113,6 +115,7 @@ class MailHandler < ActionMailer::Base
     end
     @user = User.find_by_mail(sender_email) if sender_email.present?
     if @user && !@user.active?
+      error << "Ignoring email from non-active user [#{@user.login}]."
       if logger
         logger.info  "MailHandler: ignoring email from non-active user [#{@user.login}]"
       end
@@ -134,6 +137,7 @@ class MailHandler < ActionMailer::Base
             ::Mailer.deliver_account_information(@user, @user.password)
           end
         else
+          error << "Could not create account for [#{sender_email}]."
           if logger
             logger.error "MailHandler: could not create account for [#{sender_email}]"
           end
@@ -141,6 +145,7 @@ class MailHandler < ActionMailer::Base
         end
       else
         # Default behaviour, emails from unknown users are ignored
+        error << "Ignoring email from unknown user [#{sender_email}]."
         if logger
           logger.info  "MailHandler: ignoring email from unknown user [#{sender_email}]"
         end
@@ -148,7 +153,7 @@ class MailHandler < ActionMailer::Base
       end
     end
     User.current = @user
-    dispatch
+    dispatch(error)
   end
 
   private
@@ -157,7 +162,7 @@ class MailHandler < ActionMailer::Base
   ISSUE_REPLY_SUBJECT_RE = %r{\[(?:[^\]]*\s+)?#(\d+)\]}
   MESSAGE_REPLY_SUBJECT_RE = %r{\[[^\]]*msg(\d+)\]}
 
-  def dispatch
+  def dispatch(error)
     headers = [email.in_reply_to, email.references].flatten.compact
     subject = email.subject.to_s
     if headers.detect {|h| h.to_s =~ MESSAGE_ID_RE}
@@ -177,12 +182,15 @@ class MailHandler < ActionMailer::Base
     end
   rescue ActiveRecord::RecordInvalid => e
     # TODO: send a email to the user
+    error << "#{e.message}."
     logger.error "MailHandler: #{e.message}" if logger
     false
   rescue MissingInformation => e
+    error << "Missing information from #{user}: #{e.message}."
     logger.error "MailHandler: missing information from #{user}: #{e.message}" if logger
     false
   rescue UnauthorizedAction => e
+    error << "Unauthorized attempt from #{user}."
     logger.error "MailHandler: unauthorized attempt from #{user}" if logger
     false
   end
diff --git extra/mail_handler/rdm-mailhandler.rb extra/mail_handler/rdm-mailhandler.rb
index 901f7bf..1e7777e 100644
--- extra/mail_handler/rdm-mailhandler.rb
+++ extra/mail_handler/rdm-mailhandler.rb
@@ -176,8 +176,8 @@ END_DESC
              "Make sure that 'WS for incoming emails' is enabled in application settings and that you provided the correct API key."
         return 77
       when 422
-        warn "Request was denied by your Redmine server. " +
-             "Possible reasons: email is sent from an invalid email address or is missing some information."
+        warn response.body
+        warn "Request was denied by your Redmine server."
         return 77
       when 400..499
         warn "Request was denied by your Redmine server (#{response.code})."