Using awk/sed to flip around some email address?

try:

awk -F'@' '
{ split($2, flip, "."); 
  for (i=length(flip); i>=1; i--) printf flip[i] (i!=1?".":" "); 
  print $0;
}' infile

• define @ as field delimiter with -F'@'
• split the second field on dot . separator to an array called flip
• loop over elements of array from last to first and printf each and print back . except for first element; then print whole line $0.

note: for the awk that doesn't support array_length ( see AWK - How to count stored or index on array), try below instead that is first finding how many elements the array taken and use it as max in for-loop, like:

awk -F'@' '
{ split($2, flip, ".");
  max=i=0; for (elements in flip) max++;
  for (i=max; i>=1; i--) printf flip[i] (i!=1?".":" ");
  print $0;
}' infile

You can do this perl if you insist on doing this in one line. Basically the -F flag is the same as awk so it splits each line on the @ character. The first part of the one liner creates a variable named $s that has the reversed part of the domain. The second part of the one liner prints out the reversed domain followed by the original input that is stored in the $_ variable.

perl -F'@ ' -lane '$s = join ".", reverse split/\./, $F[-1]; print "$s $_"'

#sed -r -n 's/^([^@]+@)(.+)\.([a-z]{2,3})[\r\n\t ]{0,}$/\3.\2 \1\2.\3/gip'  <<< "email: [email protected]"

or

#sed -r -n 's/^([^@]+@)(.+)\.([a-z]{2,3})[\r\n\t ]{0,}$/\3.\2 \1\2.\3/gip'  ./your file

---

upd:FIXED TO SUPPORT A THIRD DOMAIN

sed -r -n 's/^([^@]+@)([^\.]+)(\.[^\.]+){0,1}\.([a-z]{2,3})[\r\n\t ]{0,}$/\4\3.\2 \1\2\3.\4/gip'  <<< "email: [email protected]"
result: ru.yandex email: [email protected]

and

sed -r -n 's/^([^@]+@)([^\.]+)(\.[^\.]+){0,1}\.([a-z]{2,3})[\r\n\t ]{0,}$/\4\3.\2 \1\2\3.\4/gip'  <<< "email: [email protected]"
result: com.changeip.josephay905s email: [email protected]

Thanks for the comment @TERDON