It is not rare that Microsoft Outlook stops synchronizing messages due to the corruption of its data files (<database name>.PST or <database name>.OST). Here are some symptoms that could indicate a data  file corruption:

• Outlook crashes.
• Unable to synchronize messages.
• Outlook does not work after restarting.
• The Outlook data file cannot be accessed/opened.
• The set of mailbox folders cannot be opened.
• Outlook Items, such as email messages, contacts, notes, calendar items, and other stored contents, become inaccessible.

In such situations, our recommendation is to leave the current Outlook profile as it is and create a new one from scratch by following these steps:

Step 1

On Microsoft Outlook's menu, click File -> Account Settings -> Manage Profiles:
 

Step 2

 
Click "Show Profiles..." and then "Add":
 

 

Step 3

 
Inform a name for the new profiles, such as "New Outlook Profile", and click OK.

Step 4

 
A window called "Add Account" will be displayed. Fill in the fields with your email account information and click "Next":
 

Step 5

 
Check the "Change account settings" checkbox and click "Next":

Step 6

On the next screen, (1) ensure the fields are filled in appropriately. When finished, (2) click "More Settings...":
 
 

Step 7

The "Internet Email Settings" window will be displayed. On the "Outgoing Server" tab, check the option "My outgoing server (SMTP) requires authentication":
 

Step 8

On the same windows, click the "Advanced" tab and make sure the settings match the ones below. 
 

For the "Outgoing server (SMTP)" port and encryption settings, the following should be used:

• For the "Plesk" hosting, use port "465" with "SSL/TLS" encryption.
• For the "CloudMail" hosting, use port "587" with "STARTTLS" encryption.

When finished, click OK:

Step 9

Click "Finish". 

Step 10

Wait for the process to finish. When a window similar to the one below is displayed, click "Close":

Step 11

Back to the "Mail" window, click on the drop-down box under "Always use this profile", and select the profile you just created, as in the example below:
 

Then, click OK to continue.

Step 12

 Close Microsoft Outlook completely and re-open it. The email message synchronization should start.
 

Reverting the changes

In the unlikely event of anything going wrong, it is always possible to switch back to the old profile by following the steps below:

Undo Step 1

On Microsoft Outlook's menu, click File -> Account Settings -> Manage Profiles:

Undo Step 2

 The "Mail Setup" dialogue box will appear. Click "Show Profiles":

Undo Step 3

Under "Always use this profile", select the previous profile (usually "Outlook") and click OK.

Undo Step 4

Restart Microsoft Outlook.