After each mail is sent, you can re-initialize the rich text field And the mail document using Delete to get rid of the existing object. Each entry into the loop should use New (or CreateXXX) to get a new object:
For i = 1 To 6
Set maildoc = db.CreateDocument
. ‘Set subject, recipient fields
Set rtBody = New NotesRichTextItem(maidoc, “Body”)
. ‘append message text, doclinks, etc.