Email Errors from Spammers

[David Baron <d_baron@012.net.il>]

Recently, I have been getting a load of errors from fetchmail of the form:
May 13 19:01:15 ... fetchmail[6102]: SMTP error: 501 
<fedexcouriercompany111.yahoo.com@[98.110.119.254]>: domain literals not 
allowed.
This email is spam.

There are also loads of errors from a small number of senders like:
2010-05-13 15:06:34 SMTP call from localhost  ... dropped: too many syntax or 
protocol errors (last command was "MAIL 
FROM:<jameswellington000.org@[71.121.223.194]> SIZE=8814")
This is also most definitely spam.

To keep chasing these down and attempting to blacklist up front if this can be 
done would be a lot of bother. Any other ideas? (Or simply let the logs fill 
up with this harmless junk and save spamassassin the bother :-) ??)