diff --git a/Mailnag/backends/imap.py b/Mailnag/backends/imap.py
index b4d1540..d72de76 100644
--- a/Mailnag/backends/imap.py
+++ b/Mailnag/backends/imap.py
@@ -144,11 +144,15 @@ class IMAPMailboxBackend(MailboxBackend):
 			
 			for m in sorted_mails:
 				if ('uid' in m.flags) and ('folder' in m.flags):
-					folder = m.flags['folder']
-					if folder != last_folder:
-						conn.select(f'"{folder}"', readonly = False)
-						last_folder = folder
-					status, data = conn.uid("STORE", m.flags['uid'], "+FLAGS", "(\Seen)")
+					try:
+						folder = m.flags['folder']
+						if folder != last_folder:
+							conn.select(f'"{folder}"', readonly = False)
+							last_folder = folder
+						status, data = conn.uid("STORE", m.flags['uid'], "+FLAGS", "(\Seen)")
+					except:
+						logging.warning("Failed to set mail with uid %s to seen on server (account: '%s').", m.flags['uid'], acc.name)
+
 		finally:
 			self._disconnect(conn)
 	
diff --git a/Mailnag/common/config.py b/Mailnag/common/config.py
index fcbdf2d..433a81c 100644
--- a/Mailnag/common/config.py
+++ b/Mailnag/common/config.py
@@ -1,4 +1,4 @@
-# Copyright 2011 - 2019 Patrick Ulbrich <zulu99@gmx.net>
+# Copyright 2011 - 2020 Patrick Ulbrich <zulu99@gmx.net>
 #
 # This program is free software; you can redistribute it and/or modify
 # it under the terms of the GNU General Public License as published by
@@ -23,11 +23,12 @@ from configparser import RawConfigParser
 mailnag_defaults = {
 	'core':
 	{
-		'poll_interval'		: '10',
-		'imap_idle_timeout'	: '10',
-		'autostart'			: '1',
-		'connectivity_test'	: 'auto',
-		'enabled_plugins'	: 'dbusplugin, soundplugin, libnotifyplugin'
+		'poll_interval'		 : '10',
+		'imap_idle_timeout'	 : '10',
+		'mailbox_seen_flags' : '1',
+		'autostart'			 : '1',
+		'connectivity_test'	 : 'auto',
+		'enabled_plugins'	 : 'dbusplugin, soundplugin, libnotifyplugin'
 	}
 }
 
diff --git a/Mailnag/daemon/mailchecker.py b/Mailnag/daemon/mailchecker.py
index fff9c5b..6970179 100644
--- a/Mailnag/daemon/mailchecker.py
+++ b/Mailnag/daemon/mailchecker.py
@@ -32,6 +32,7 @@ class MailChecker:
 		self._firstcheck = True # first check after startup
 		self._mailcheck_lock = threading.Lock()
 		self._mailsyncer = MailSyncer(cfg)
+		self._mailbox_seen_flags = bool(cfg.get('core', 'mailbox_seen_flags'))
 		self._memorizer = memorizer
 		self._hookreg = hookreg
 		self._conntest = conntest
@@ -63,7 +64,7 @@ class MailChecker:
 						unseen_mails.append(mail)
 						if self._firstcheck:
 							new_mails.append(mail)
-					else:
+					elif self._mailbox_seen_flags:
 						# if the mail account supports tagging mails as seen (e.g. IMAP), 
 						# mark the mail as seen on the server as well.
 						if mail.account.supports_mark_as_seen():
@@ -76,10 +77,7 @@ class MailChecker:
 			
 			# Flag mails to seen on server
 			for acc, mails in seen_mails_by_account.items():
-				try:
-					acc.mark_as_seen(mails)
-				except:
-					logging.warning("Failed to set mails to seen on server (account: '%s').", acc.name)
+				acc.mark_as_seen(mails)
 			
 			self._memorizer.sync(all_mails)
 			self._memorizer.save()