Thanks, everyone, for the help. Stan's advice that these "are probably not
worth obsessing about" is definitely worth listening to! :-> Regarding
Bill's comment:

> On many systems, a maximum message size can be defined.  I beleive Hotmail
> is about 1.5 MB.

At least one of the bouncing messages was less than 30k, so I've come to
believe Microsoft uses 5.2.3 to mean they have disabled the account by
setting the quota to zero.

     peter