Some nice grep tricks:

search for mail adresses within a given file:
grep -E -o "\b[a-zA-Z0-9.-._]+@[a-zA-Z0-9.-]+\.[a-zA-Z0-9.-]+\b"

Search for multiple words given in a file with one keyword each line:
cat inputfile.txt | grep -Ff file_with_keywords.txt > results.txt

Search multiple words given by command line:
cat inputfile.txt | grep -iE "(keywords1|keyword2|keyword2)" > results.txt