Hi,
yesterday I had issues, sendmail was not responding quickly while sending email and users did not receive any email,
This happened with sendmail on solaris 10 and also with sendmail on a linux box.
Found it is something to do with DNS blacklists,
following lists did not work for me,
now this could be a known issue, but why so many blacklists servers stopped suddenly 
Again, is there any solution on this , like some config we can do in sendmail to work around.
I have removed the lists not responding to nslookup but the ones which are already here could also stop responding some day
by mentioning the blacklists here, I hope it helps others also :)
Thanks!