MailMerge.MailFormat (Word)

Returns a WdMailMergeMailFormat constant that represents the format to use when the mail merge destination is an email message. Possible return values are wdMailFormatHTML - Sends mail merge email documents using HTML format, wdMailFormatPlainText - Sends mail merge email documents using plain text.

The MailFormat property is ignored if the MailAsAttachment property is set to True. Conversely, when MailFormat is set, MailAsAttachment is automatically set to False.

Sub MergeDestination() 
    With ActiveDocument.MailMerge 
        .Destination = wdSendToEmail 
        .MailAsAttachment = False 
        .MailFormat = wdMailFormatHTML 
        .Execute 
    End With 
End Sub