my users are getting more spam mails, every time mail with new header and subject and incoming domain change, and specific size of attachment 74k every time new name of attachment .zip file.
can any one help me according this, i am also using spam-assisan with sendmail.

The mail message you are recieving sounds more like a virus that spam.
Having a *.zip attachment with a changing name and the same size as well
as the other aspect of the message say to me that it is an attempt to
send you a virus. The attachment would be the virus and if someone save it
and tries to unzip the file the virus will then affect the system.
I do not garauntee that this is what is going on it just seems the most
likely given the information you provided. If it is the case that the
message is an attempt at a virus attack, spam-assisian wil not catch it
but an anit-virus software such as Clam-AV will.