With `sed` alone:
sed -i '/^* !^FROM_MAILER$/d' file
To remove the whole line containing the exact string `* !^FROM_MAILER`, with nothing before and after that string. The `d` command deletes the line.
* * *
**Edit** : If you want to do the replace in all files recusively, use the following:
find /path -type f -exec sed -i '/^* !^FROM_MAILER$/d' {} +