Recently I made the switch from Google Gmail to Microsoft Outlook for my personal emails. This migration also meant I had to move over all of my existing emails from Google to Microsoft. In order to migrate the whole mailbox from one account provider to the other, I used Microsoft Outlook for Mac device running the latest macOS operating system and Outlook 16.6 (latest version at the time of writing this blogpost).

Unfortunately, some of the mails that were migrated showed up in the Sync Error dialog and reported some kind of error message. There error messages contained the information about the mail (the subject line), the time at then the error in the synchronisation happened, the error message itself, the details of this error message and some kind of cryptic error code. Here are some examples of error messages that I have observed:

• Error: Could not synchronise record. Operation would change object type, which is not permitted. Error code: -19703
• Error: Could not synchronise record. ChangeKey is required for this operation. Error code: -19928
• Error: Could not synchronize record. Property is not valid for this object type. Error code: -19803

Solution 1: empty the cache

In my case, the migration of my mails meant that over 100.000 emails were transferred from one mailbox to another in a matter of hours. The Outlook program has to orchestrate the migration and there is always a chance something is going wrong with the cache of the Outlook mail program. You should try at least to empty the cache and see if the issue remains

1. Ensure you don’t work in Off-line mode first.
2. In the navigation pane, right-click the Exchange folder for which you want to empty the cache, and then click Properties.
3. On the General tab, click Empty Cache. The folder will be emptied, so be prepared because Outlook will automatically download all the email items again from the Exchange server.

Try also to restart Microsoft Outlook at this point (fully quit the application and open it again).

Solution 2: Delete these items manually

Typically, the affected items are containing special characters, or they are out of office (automatic replies), or RSVP calendar replies or any other special types of email which blocks the synchronisation process.

In this case, given the number of emails that fail to synchronise, it may be advisable to manually look up these emails and remove the manually. Also ensure to remove them from the bin / deleted items folder because the error message may still persist otherwise.