How to use SED or AWK to search and replace an exact string

I have a file

DS1
DDS
DS

I want to replace only "DS" to "DSmail.blah.com" in a lot of files. I tried

sed 's/DS/DSmail.blah.com' but it changes all the lines .

thanks in advance

sample:

 
$sed 's/^DS$/abc/' test
DS1
DDS
abc

 
perl -pe 's/\bDS\b/blah/' input
DS1
DDS
blah