diff --git a/app/Gulag.py b/app/Gulag.py
index 8167db1..c1cd43a 100644
--- a/app/Gulag.py
+++ b/app/Gulag.py
@@ -1,4 +1,4 @@
-import json,sys
+import json,sys,os,logging
 import email,email.header,email.message
 from GulagDB import GulagDB,GulagDBException
 from GulagMailbox import IMAPmailbox,IMAPmailboxException
@@ -23,13 +23,32 @@ class Gulag:
       f.close()
     except:
       raise GulagException(whoami(self) + str(sys.exc_info()))
+    # logging
+#    logging_level = logging.INFO
+#    if 'level' in self.config['logging']:     
+    if 'logging' not in self.config:
+      raise GulagException(whoami(self) + "Logging not configured!")
+    if('filename' in self.config['logging'] and 
+       len(self.config['logging']['filename']) > 0):
+      logging.basicConfig(
+        filename=self.config['logging']['filename'],
+        format='%(asctime)s %(levelname)s %(message)s',
+        level=self.config['logging']['level']
+      )
+    else:
+      logging.basicConfig(
+        format='%(asctime)s %(levelname)s %(message)s',
+        level=self.config['logging']['level']
+      )
     try:
       self.db = GulagDB(self.config['db'],self.config['uri_prefixes'])
       self.fields['Mailboxes'] = self.db.get_fields('Mailboxes')
       self.fields['QuarMails'] = self.db.get_fields('QuarMails')
       self.fields['Attachments'] = self.db.get_fields('Attachments')
     except GulagDBException as e:
+      logging.warning(whoami(self) + e.message)
       raise GulagException(whoami(self) + e.message) from e
+    logging.info('Gulag core initialized by ' + os.path.basename(__file__))
    
   def check_fields(self,fields_target,args):
     if fields_target not in self.fields:
@@ -54,7 +73,7 @@ class Gulag:
       try:
         imap_mb = IMAPmailbox(mailbox)
       except IMAPmailboxException as e:
-        print(e.message)
+        logging.warning(whoami(self) + e.message)
         continue
       for unseen in imap_mb.get_unseen_messages():
         quarmail_ids = []
@@ -70,13 +89,17 @@ class Gulag:
         try:
           r5321_rcpts = email.header.decode_header(msg['X-Envelope-To-Blocked'])[0][0]
         except:
-          print("Failed to extract envelope recipients! Skipping mail")
+          logging.warning(whoami(self) + 
+            "Failed to extract envelope recipients! Skipping mail"
+          )
           continue
         r5322_from = None
         try:
           r5322_from = email.header.decode_header(msg['From'])[0][0]
         except:
-          print("Failed to extract from header! Skipping mail")
+          logging.warning(whoami(self) + 
+            "Failed to extract from header! Skipping mail"
+          )
           continue
         subject = email.header.decode_header(msg['Subject'])[0][0]
         msg_id = None
@@ -109,7 +132,7 @@ class Gulag:
             'hdr_msgid': msg_id, 'hdr_date': date, 'cf_meta': 'cf_meta',
             'mailbox_id': 'quarantine@zwackl.de', 'imap_uid': uid, 'msg_size': msg_size
           })
-          print("QuarMail (%s) imported" % (quarmail_id))
+          logging.info(whoami(self) + "QuarMail (%s) imported" % (quarmail_id))
           quarmail_ids.append(quarmail_id)
         # Ende for rcpts
         # Alle MIME-Parts durchiterieren und Attachments
@@ -142,16 +165,15 @@ class Gulag:
     # Ende for get_mailboxes
 
   def cleanup_quarmails(self):
-    print("Mails to expunge: " + 
-      str(len(
-        self.db.get_deprecated_mails(self.config['cleaner']['retention_period'])
-      ))
-    )
+    logging.info(whoami(self) + "QuarMails to purge:  " + str(len(
+      self.db.get_deprecated_mails(self.config['cleaner']['retention_period'])
+     )))
   
   def get_mailboxes(self):
     try:
       return self.db.get_mailboxes()
     except GulagDBException as e:
+      logging.warning(whoami(self) + e.message)
       raise GulagException(whoami(self) + e.message) from e
 
   def get_quarmails(self,args):
@@ -159,6 +181,7 @@ class Gulag:
       self.check_fields('QuarMails',args)
       return self.db.get_quarmails(args)
     except(GulagException,GulagDBException) as e:
+      logging.warning(whoami(self) + e.message)
       raise GulagException(
         whoami(self) + e.message
       ) from e
@@ -168,6 +191,7 @@ class Gulag:
     try:
       qm_db = self.db.get_quarmail({"id": args['quarmail_id']})
     except GulagDBException as e:
+      logging.warning(whoami(self) + e.message)
       raise GulagException(whoami(self) + e.message) from e
     if 'rfc822_message' not in args:
       return qm_db
@@ -176,6 +200,7 @@ class Gulag:
     try:
       mailbox = self.db.get_mailbox(qm_db['mailbox_id'])
     except GulagDBException as e:
+      logging.warning(whoami(self) + e.message)
       raise GulagException(whoami(self) + e.message) from e 
     imap_mb = None
     try:
@@ -183,14 +208,14 @@ class Gulag:
       qm_db['rfc822_message'] = imap_mb.get_message(qm_db['imap_uid']).decode("utf-8")
       return qm_db
     except IMAPmailboxException as e:
-      print(e.message)
+      logging.warning(whoami(self) + e.message)
       raise GulagException(whoami(self) + e.message) from e
 
   def get_quarmail_attachments(self,args):
     try:
       return self.db.get_quarmail_attachments(args['quarmail_id'])
     except GulagDBException as e:
-      print(e.message)
+      logging.warning(whoami(self) + e.message)
       raise GulagException(whoami(self) + e.message) from e
   
   def get_quarmail_attachment(self,args):
@@ -200,7 +225,7 @@ class Gulag:
         args['quarmail_id'],args['attachment_id']
       )
     except GulagDBException as e:
-      print(e.message)
+      logging.warning(whoami(self) + e.message)
       raise GulagException(whoami(self) + e.message) from e
     if 'data' not in args:
       return qmat_db
@@ -209,6 +234,7 @@ class Gulag:
     try:
       mailbox = self.db.get_mailbox(qmat_db['mailbox_id'])
     except GulagDBException as e:
+      logging.warning(whoami(self) + e.message)
       raise GulagException(whoami(self) + e.message) from e 
     imap_mb = None
     try:
@@ -218,7 +244,7 @@ class Gulag:
       )
       return qmat_db
     except IMAPmailboxException as e:
-      print(e.message)
+      logging.warning(whoami(self) + e.message)
       raise GulagException(whoami(self) + e.message) from e
 
   def get_attachment(self,args):
@@ -243,19 +269,52 @@ class Gulag:
     # check if the request comes really from rspamd´s metadata_exporter
     # default metadata_header prefix 'X-Rspamd' will be expected
     if('X-Rspamd-From' not in args['req_headers']):
-      raise GulagException(whoami(self) 
-        + "Missing Rspamd-specific headers (e.g. X-Rspamd-From)!"
+      err = str(whoami(self) 
+        + "Missing Rspamd-specific request header X-Rspamd-From!"
       )
+      logging.error(err)
+      raise GulagException(err)
     # Prepend Gulag-specific headers to rejected mail 
     # before pushing into quarantine mailbox
     msg = None
     try:
+      if('X-Rspamd-From' not in args['req_headers']):
+        err = str(whoami(self) 
+          + "Missing Rspamd-specific request header X-Rspamd-From!"
+        )
+        logging.error(err)
+        raise GulagException(err)
+      if('X-Rspamd-Rcpt' not in args['req_headers']):
+        err = str(whoami(self) 
+          + "Missing Rspamd-specific request header X-Rspamd-Rcpt!"
+        )
+        logging.error(err)
+        raise GulagException(err)
+      if('X-Rspamd-Symbols' not in args['req_headers']):
+        err = str(whoami(self) 
+          + "Missing Rspamd-specific request header X-Rspamd-Symbols!"
+        )
+        logging.error(err)
+        raise GulagException(err)
+      if('X-Rspamd-Qid' not in args['req_headers']):
+        err = str(whoami(self) 
+          + "Missing Rspamd-specific request header X-Rspamd-Qid!"
+        )
+        logging.error(err)
+        raise GulagException(err)
+      if('rfc822_message' not in args['rfc822_message']):
+        err = str(whoami(self) 
+          + "Missing rfc822_message!"
+        )
+        logging.error(err)
+        raise GulagException(err)
+      # all mandatory request headers and body are present
       rcpts_hdr = ""
       for rcpt in json.loads(str(args['req_headers']['X-Rspamd-Rcpt'])):
         if(len(rcpts_hdr) > 0):
           rcpts_hdr += "," + rcpt
         else:
-          rcpts_hdr = rcpt
+          rcpts_hdr = rcpt 
       msg = "Return-Path: <" + args['req_headers']['X-Rspamd-From'] + ">\r\n"
       msg += "Received: from rspamd_http2imap relay by gulag-mailbox " 
       msg += args['mailbox_id'] + "\r\n"
@@ -264,6 +323,8 @@ class Gulag:
       msg += "X-Spam-QID: " + args['req_headers']['X-Rspamd-Qid'] + "\r\n"
       # append original mail
       msg += args['rfc822_message']
+    except GulagException as e:
+      raise GulagException(e.message) from e
     except:
       raise GulagException(whoami(self) + str(sys.exc_info()))
     imap_mb = None
diff --git a/app/GulagUtils.py b/app/GulagUtils.py
index 63b81de..cc9e05c 100644
--- a/app/GulagUtils.py
+++ b/app/GulagUtils.py
@@ -1,7 +1,6 @@
 import sys
 from smtplib import SMTP
 
-
 def whoami(obj):
   return type(obj).__name__ + "::" + sys._getframe(1).f_code.co_name + "(): "
 
diff --git a/app/Resources.py b/app/Resources.py
index 4e5efbd..ba5c8e8 100644
--- a/app/Resources.py
+++ b/app/Resources.py
@@ -103,6 +103,7 @@ class ResRSPAMDImporter(GulagResource):
         "rfc822_message": request.get_data(as_text=True)
       })
       # TODO: Response mit Location-Header?
+      # https://stackoverflow.com/a/22707491
       return {"resource: ": "HTTP2IMAP for RSPAMD"}
     except GulagException as e:
       abort(400, message=e.message)
diff --git a/config/gulag-config.json b/config/gulag-config.json
index b07b300..1f906c9 100644
--- a/config/gulag-config.json
+++ b/config/gulag-config.json
@@ -1,4 +1,10 @@
 {
+  "logging": {
+    "__level": "default: WARNING. Possible: INFO,ERROR,CRITICAL,DEBUG",
+    "level": "DEBUG",
+    "__filename": "default: empty string (stdout).",
+    "filename": ""
+  },
   "daemon":{
     "listen_host": "127.0.0.1",
     "listen_port": 5001
@@ -17,10 +23,10 @@
     }
   },
   "uri_prefixes": {
-    "root": "https://gulag.example.org/api/v1/",
-    "mailboxes": "https://gulag.example.org/api/v1/mailboxes/",
-    "quarmails": "https://gulag.example.org/api/v1/quarmails/",
-    "attachments": "https://gulag.example.org/api/v1/attachments/"
+    "root": "http://127.0.0.1:9090/api/v1/",
+    "mailboxes": "http://127.0.0.1:9090/api/v1/mailboxes/",
+    "quarmails": "http://127.0.0.1:9090/api/v1/quarmails/",
+    "attachments": "http://127.0.0.1:9090/api/v1/attachments/"
   },
   "db":{
     "unix_socket": "/mysqld/mysqld.sock",