Fetchmail/Sendmail rejects

[Malcolm Kay]

I run fetchmail in daemon mode to download
POP3 mail from my ISP.

Sendmail rejects many messages as for example:
Aug 14 16:59:33 beta sm-mta[35000]: i7E7DYje035000: 
  ruleset=check_mail, arg1=<fbbjqwvcydimv at lightspeedcredit.com>, 
  relay=localhost.home [127.0.0.1], reject=451 
  4.1.8 Domain of sender address fbbjqwvcydimv at lightspeedcredit.com 
  does not resolve

I am quite happy to have these rejected but it seems they don't 
get deleted at the ISP end and clog up the mail box, I think 
eventually confusing fetchmail.

Is there some reasonable way of disposing of these messages. I would
prefer not to download these into my normal user mailbox but I would 
be quite happy to divert them to some pseudo user setup for the 
purpose. I would imagine sendmail can be coaxed into doing this, but
how?

Responding to 2 or 3 lists using a valid e-mail address means that
I receive a lot of spam including much with unresolvable addresses.

I would appreciate any ideas.

Malcolm