A Document_Open macro runs when you open a document. By definition, that document is both the active document and takes the foreground. When a macro executes a mailmerge, the output document becomes the active document. To see how you might go about implementing a mailmerge via a macro, see
Send Mailmerge Output to Individual Files in the
Mailmerge Tips and Tricks 'Sticky' thread at:
https://www.msofficeforums.com/mail-...ps-tricks.html