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]