Same problem here (just downloaded it now):
[root@zimbra ~]# wget
http://h.yimg.com/lo/downloads/5.0.1...0707141910.tgz
--07:32:40--
http://h.yimg.com/lo/downloads/5.0.1...0707141910.tgz
=> `zcs-NETWORK-5.0.18_GA_3012.RHEL4.20090707141910.tgz'
Resolving h.yimg.com... 8.12.202.126, 199.93.63.126, 209.84.30.126
Connecting to h.yimg.com|8.12.202.126|:80... connected.
HTTP request sent, awaiting response... 200 OK
Length: 519,421,952 (495M) [binary/octet-stream]
100%[================================================== ======================================>] 519,421,952 6.65M/s ETA 00:00
07:33:59 (6.23 MB/s) - `zcs-NETWORK-5.0.18_GA_3012.RHEL4.20090707141910.tgz' saved [519421952/519421952]
[root@zimbra ~]# wget
http://h.yimg.com/lo/downloads/5.0.1...141910.tgz.md5
--07:38:51--
http://h.yimg.com/lo/downloads/5.0.1...141910.tgz.md5
=> `zcs-NETWORK-5.0.18_GA_3012.RHEL4.20090707141910.tgz.md5'
Resolving h.yimg.com... 198.78.215.126, 199.93.63.126, 204.160.104.126
Connecting to h.yimg.com|198.78.215.126|:80... connected.
HTTP request sent, awaiting response... 200 OK
Length: 86 [binary/octet-stream]
100%[================================================== ======================================>] 86 --.--K/s
07:38:52 (6.35 MB/s) - `zcs-NETWORK-5.0.18_GA_3012.RHEL4.20090707141910.tgz.md5' saved [86/86]
[root@zimbra ~]# md5sum -c zcs-NETWORK-5.0.18_GA_3012.RHEL4.20090707141910.tgz.md5
zcs-NETWORK-5.0.18_GA_3012.RHEL4.20090707141910.tgz: FAILED
md5sum: WARNING: 1 of 1 computed checksum did NOT match