Sed Command Giving Carriage Return

It appears that your original file test.csv is Windows-formatted, with return (0x0d or \r) and line feed (0x0a or\n) for each new-line. Your sed search pattern does not exclude \r so this will be retained in the second matched field and output before the appended text.

So your original command:

sed -i .bak -E 's/([^,\n\“]*)@([^,\n\“]*)/\1_X_\2@ Test.csv

gives the output file (using od -c to see the control characters):

0000000   f   o   o   _   X   _   e   x   a   m   p   l   e   .   c   o
0000020   m  \r   @   n   e   w   d   o   m   a   i   n   .   c   o   m
0000040  \n

If you add \r to the excluded characters:

sed -i .bak -E 's/([^,\r\n\“]*)@([^,\r\n\“]*)/\1_X_\2@ Test.csv

then the original new-line sequence is maintained:

0000000   f   o   o   _   X   _   e   x   a   m   p   l   e   .   c   o
0000020   m   @   n   e   w   d   o   m   a   i   n   .   c   o   m  \r
0000040  \n

Note that you don't need \r or \n in the first matching field, as this form of sed command will match only within single lines and not through new-lines.