Large number of user unknown entries

I'm getting tons of user unknown entries from all kinds of relay servers trying to send to non existent users in my domain, below is a snippet of my maillog file (mydomain.com being my domain).  I'm not sure if its some kind of indirect DDoS attack, whereby someone's computer was infected and starting sending emails to different users at different domains with a return address of a random user at my domain.  I just started getting these today that I'm aware of, regular mail is still passing OK at the moment.  I know you're not supposed to block blank from addresses because they are used for bounces, but I'm not sure how else to stop these because they are coming from everywhere.

I am using sendmail 8.13 with ClamAV and spamassassin.

Apr 14 11:56:11 mail sendmail[3526]: o3EFuAjd003526: from=<>, size=9005, class=0, nrcpts=0, proto=ESMTP, daemon=MTA, relay=vd1.online.sh.cn [202.96.209.52]
Apr 14 11:56:11 mail sendmail[3533]: o3EFuBEO003533: <epepicani1383@mydomain.com>... User unknown
Apr 14 11:56:11 mail sendmail[3533]: o3EFuBEO003533: from=<>, size=11007, class=0, nrcpts=0, proto=ESMTP, daemon=MTA, relay=smtp-in-109.livemail.co.uk [213.171.216.170]
Apr 14 11:56:13 mail sendmail[3540]: o3EFuCLK003540: <aeledi6024@mydomain.com>... User unknown
Apr 14 11:56:13 mail sendmail[3540]: o3EFuCLK003540: from=<PostMaster@qq.com>, size=0, class=0, nrcpts=0, proto=SMTP, daemon=MTA, relay=[64.71.138.53]
Apr 14 11:56:14 mail sendmail[3543]: STARTTLS=server, relay=relay03.mail.esat.net [193.95.141.41], version=TLSv1/SSLv3, verify=NO, cipher=AES256-SHA, bits=256/256
Apr 14 11:56:14 mail sendmail[3543]: o3EFuDhm003543: <ymisyo6047@mydomain.com>... User unknown
Apr 14 11:56:14 mail sendmail[3543]: o3EFuDhm003543: from=<>, size=9720, class=0, nrcpts=0, proto=ESMTP, daemon=MTA, relay=relay03.mail.esat.net [193.95.141.41]
Apr 14 11:56:15 mail sendmail[3548]: o3EFuFAO003548: <abialixal7293@mydomain.com>... User unknown
Apr 14 11:56:15 mail sendmail[3548]: o3EFuFAO003548: from=<>, size=9872, class=0, nrcpts=0, proto=ESMTP, daemon=MTA, relay=popserv.tcp.net.uk [195.80.0.247]
Apr 14 11:56:16 mail sendmail[3553]: STARTTLS=server, relay=mx3.freeparking.co.uk [72.1.194.110], version=TLSv1/SSLv3, verify=NO, cipher=DHE-RSA-AES256-SHA, bits=256/256
Apr 14 11:56:16 mail sendmail[3553]: o3EFuFXU003553: <uuqaje2242@mydomain.com>... User unknown
Apr 14 11:56:16 mail sendmail[3553]: o3EFuFXU003553: from=<>, size=7542, class=0, nrcpts=0, proto=ESMTP, daemon=MTA, relay=mx3.freeparking.co.uk [72.1.194.110]
sutech08Asked:
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Jan SpringerCommented:
I think that this is either a spam attempt to any possible address or, more likely, someone trying to determine valid accounts.

Your sendmail.mc should have some combination of these configurations:

  define(`confPRIVACY_FLAGS', `authwarnings,novrfy,noexpn,restrictqrun')dnl
  define(`confBAD_RCPT_THROTTLE', `1')dnl
  define(`confMAX_RCPTS_PER_MESSAGE', `20')dnl
  define(`confCONNECTION_RATE_THROTTLE', `100')dnl
  FEATURE(`greet_pause',1500)dnl

For feature greet_pause, add in your access file:

  GreetPause:localhost    0

Connection rate throttle and max recipients per message need to be tuned based upon the size of your user base.  The '1500' in greet_pause is a 1.5 second delay between the connection and a return 220 greeting.
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