Message-ID contains "To" address and spammers finding it

I have a list where I use the archives email to start up mhonarc.  The 
message-ID tag contains the email address and spammers are finding it (I've 
been changing it over and over and it's the only place they are getting it) 
so spam is getting into our archives.
I can't seem to exclude Message-ID from the hidden tags with excs override 
or other tags.
Ideas?  Thanks.