Yes, the testpaper.com.sg. MX entries will prevent many people from being able to send mail.

[Jonathan de Boyne Pollard]

J> Is the MX mail entry using IP address instead of a 
J> hostname valid ? 

No.  It relies upon something that isn't universally true.  

   [C:\]dnsqry /recursive mx testpaper.com.sg. | tail /5
   [127.0.0.8:0035] -> [0.0.0.0:0000] 65
   Header: 0000 1+1+0+0, R, RD|RA, query, no_error
   Question: testpaper.com.sg. IN MX
   Answer: testpaper.com.sg. IN MX 85911 10 202.157.162.196.

   [C:\]

In the spirit of RFC 1912, some proxy DNS server softwares generate implicit
"A" resource records for domain names that resemble the human-readable form of
IP addresses, preventing queries for such domain names from leaking out onto
Internet.  "dnscache" from Dan Bernstein's "djbdns" and Sam Trenholme's
MaraDNS both do this, for example, to name but two.  The people around the 
world whose SMTP Relay client softwares use those softwares for their DNS
lookups won't have trouble with "MX" resource records whose domain name data 
are such IP-address-like domain names, because the second-stage lookup for the 
IP-address-like domain name will yield an implicitly generated "A" resource 
record listing the corresponding IP address.

   [C:\]dnsqry /recursive a 202.157.162.196 | tail /5
   [127.0.0.8:0035] -> [0.0.0.0:0000] 49
   Header: 0000 1+1+0+0, R, RD|RA, query, no_error
   Question: 202.157.162.196. IN A
   Answer: 202.157.162.196. IN A 604800 202.157.162.196

   [C:\]dnsgetmx testpaper.com.sg.
   202.157.162.196

   [C:\]

However, other DNS server softwares, such as ISC's BIND and Microsoft's DNS 
server, don't have implicit resource record generation for IP-address-like 
domain names, and process queries for such domain names in the same way that 
they process queries for other domain names.  The people around the world whose
SMTP Relay client softwares use those softwares for their DNS lookups _will_ 
have trouble as a consequence of "MX" resource records containing 
IP-address-like domain names; since they will find no "A" resource records in 
the public DNS database for the IP-address-like domain name and the 
second-stage lookup will thus fail, making it impossible to locate the SMTP
Relay server.

   [C:\]dnsqry /serverip:195.117.6.25 /recursive a 202.157.162.196 | tail /4
   [195.117.6.25:0035] -> [0.0.0.0:0000] 33
   Header: 0000 1+0+0+0, R, RD|RA|AUTH, query, name_error
   Question: 202.157.162.196. IN A

   [C:\]dnsgetmx /serverip:195.117.6.25 testpaper.com.sg.
   IUZ0031: The domain name "202.157.162.196." does not exist.

   [C:\]