[exim] Block access to certain IPs

Hi,

Daily I found in rejectlog file hundreds of lines like:

2012-mm-dd hh: mm: ss fixed_login authenticator failed for
([www.xxx.yyy.zzz]) [ww.xx.yyy.zz]: 535 Incorrect authentication data
(set_id = xyz)

And in most of them the IP is the same.

How I can prevents access to EXIM of those IPs?

Thanks in advance.

Jorge Colaccini