VBA Code

The mail merge object is always available regardless of whether or not the specified document is a mail merge document
If you haven't added an envelope to the active mailmerge document then using any of the following will cause an error address, addressfromleft, addressfromtop, feedsource, returnaddress, returnaddressfromleft, returnaddressfromtop and updatedocument

Type of Document

This checks to make sure the document is not a mail merge document
The State returns the current state of the mail merge operation. Read-only WdMailMergeState.

If ActiveDocument.MailMerge.State = wdMailMergeState.wdNormalDocument Then 

End If 

This checks to make sure that the document is a main document with an attached data source.

If ActiveDocument.MailMerge.State = wdMailMergeState.wdMainAndDataSource Then 

End If 

MailMerge Object

Use the MailMerge property to return the MailMerge object.
The MailMerge object is always available regardless of whether the mail merge operation has begun.
Use the State property to determine the status of the mail merge operation.
The following example executes a mail merge if the active document is a main document with an attached data source.

If ActiveDocument.MailMerge.State = wdMailMergeState.wdMainAndDataSource Then 
    ActiveDocument.MailMerge.Execute 
End If 

The following example merges the main document with the first three data records in the attached data source and then sends the results to the printer.

Set myMerge = ActiveDocument.MailMerge 
If myMerge.State = wdMailMergeState.wdMainAndSourceAndHeader Or _ 
    myMerge.State = wdMailMergeState.wdMainAndDataSource Then 
    With myMerge.DataSource 
        .FirstRecord = 1 
        .LastRecord = 3 
    End With 
End If 
With myMerge 
    .Destination = wdMailMergeDestination.wdSendToPrinter 
    .Execute 
End With 

MailMerge.EditMainDocument

Activates the mail merge main document associated with the specified header source or data source document.
Note If the main document isn't open, an error occurs. Use the Open method if the main document isn't currently open.
This example attempts to activate the main document associated with the active data source document. If the main document isn't open, the Open dialog box is displayed, with a message in the status bar.

Sub ActivateMain() 
    On Error GoTo errorhandler 
    Documents("Data.doc").MailMerge.EditMainDocument 

    Exit Sub 

errorhandler: 
    If Err = 4605 Then StatusBar = "Main document is not open" 
    Dialogs(wdWordDialog.wdDialogFileOpen).Show 
End Sub 

MailMerge.Execute

Optional Variant. True for Microsoft Word pause and display a troubleshooting dialog box if a mail merge error is found. False to report errors in a new document.

This example executes a mail merge if the active document is a main document with an attached data source.

Set myMerge = ActiveDocument.MailMerge 
If myMerge.State = wdMailMergeState.wdMainAndDataSource Then 
   MyMerge.Execute(Pause:=False) 
End If 

With ActiveDocument.MailMerge 
   .Destination = wdMailMergeDestination.wdSendToNewDocument 
   .SuppressBlankLines = True 
   With .DataSource 
      .FirstRecord = wdMailMergeDefaultRecord.wdDefaultFirstRecord 
      .LastRecord = wdMailMergeDefaultRecord.wdDefaultLastRecord 
   End With 
   .Execute Pause:=False 
End With 

MailMerge.Check

Simulates the mail merge operation, pausing to report each error as it occurs.
This example checks the active document for mail merge errors.

ActiveDocument.MailMerge.Check