There is a very useful command in Linux, called grep. It is a utility for searching plain text files using regular expressions. You can either search files and folders as a parameter, or pipe command outputs to it, such as grep’ing a tail.
For looking for a file:
grep -r "string to find" /folder/to/look/in/
Thats the simple answer.
If you want to pipe the grep to monitor say a tail
tail -f /var/log/maillog | grep "firstname.lastname@example.org"