tsar@gmail.com
c2m_@hotmail.com.es
c72@hotmail.com.es.com;es
m7_@hotmail.es.com
cm_@hotmail.com
@pepito.com

sed -n -r '/w+@w+.w+((.w+)*)?/p' emails.txt

tsar@gmail.com
cm_@hotmail.com

c2m_@hotmail.com.es
c72@hotmail.com.es.com;es
m7_@hotmail.es.com
@pepito.com

$ sed -nr '/^[^@]+@[^.]+.coms*$/p' file
tsar@gmail.com
cm_@hotmail.com

perl -ne 'print if /^[^@]+@[^.]+.coms*$/' file

grep -P '^[^@]+@[^.]+.coms*$' file

grep -E '^[^@]+@[^.]+.coms*$' file

awk '$0~/^[^@]+@[^.]+.coms*$/' file

sed -n -r '/w+@w+.com$/p' emails.txt

tsar@gmail.com
cm_@hotmail.com

sed -n -r '/w+@w+.w+s*$/p' emails.txt