Hi, I've a recurrent problem on syncing. To solve it, I've to remove internal files (rm -rf ~/.offlineimap/) and mails in .Sent folder. With offlineimap 6.3.5-rc1, the output is the following : Account sync toto_irisa: ***** Processing account toto_irisa Copying folder structure from IMAP to Maildir Establishing connection to zimbra.inria.fr:993. Folder sync [toto_irisa]: Syncing INBOX: IMAP -> Maildir Syncing Drafts: IMAP -> Maildir Syncing Sent: IMAP -> Maildir Adding flag D to 1 messages on Sent ERROR [<InstanceLimitedThread(Folder sync [toto_irisa], started daemon 140489914951424)>]: Syncing folder .Sent [acc: toto_irisa] Error with store: UID STORE failed ERROR [<InstanceLimitedThread(Folder sync [toto_irisa], started daemon 140489914951424)>]: ERROR in syncfolder for toto_irisa folder .Sent: Traceback (most recent call last): File "/home/toto/comp/offlineimap/offlineimap/accounts.py", line 358, in syncfolder localfolder.syncmessagesto(remotefolder, statusfolder) File "/home/toto/comp/offlineimap/offlineimap/folder/Base.py", line 431, in syncmessagesto action(dstfolder, statusfolder) File "/home/toto/comp/offlineimap/offlineimap/folder/Base.py", line 388, in syncmessagesto_flags dstfolder.addmessagesflags(uids, set(flag)) File "/home/toto/comp/offlineimap/offlineimap/folder/IMAP.py", line 627, in addmessagesflags self.addmessagesflags_noconvert(uidlist, flags) File "/home/toto/comp/offlineimap/offlineimap/folder/IMAP.py", line 621, in addmessagesflags_noconvert self.processmessagesflags('+', uidlist, flags) File "/home/toto/comp/offlineimap/offlineimap/folder/IMAP.py", line 653, in processmessagesflags assert r[0] == 'OK', 'Error with store: ' + '. '.join(r[1]) AssertionError: Error with store: UID STORE failed Error with store: UID STORE failed Syncing Trash: IMAP -> Maildir Account sync toto_irisa: ***** Finished processing account toto_irisa ERROR: Exceptions occurred during the run! ERROR: Syncing folder .Sent [acc: toto_irisa] Error with store: UID STORE failed ERROR: ERROR in syncfolder for toto_irisa folder .Sent: Traceback (most recent call last): File "/home/toto/comp/offlineimap/offlineimap/accounts.py", line 358, in syncfolder localfolder.syncmessagesto(remotefolder, statusfolder) File "/home/toto/comp/offlineimap/offlineimap/folder/Base.py", line 431, in syncmessagesto action(dstfolder, statusfolder) File "/home/toto/comp/offlineimap/offlineimap/folder/Base.py", line 388, in syncmessagesto_flags dstfolder.addmessagesflags(uids, set(flag)) File "/home/toto/comp/offlineimap/offlineimap/folder/IMAP.py", line 627, in addmessagesflags self.addmessagesflags_noconvert(uidlist, flags) File "/home/toto/comp/offlineimap/offlineimap/folder/IMAP.py", line 621, in addmessagesflags_noconvert self.processmessagesflags('+', uidlist, flags) File "/home/toto/comp/offlineimap/offlineimap/folder/IMAP.py", line 653, in processmessagesflags assert r[0] == 'OK', 'Error with store: ' + '. '.join(r[1]) AssertionError: Error with store: UID STORE failed Error with store: UID STORE failed Of course, I can give you more detailled information if you need it. Dal.