comment line starting with string and the next
sed '/^mail/,+1s/^/#/' /etc/passwd
comment line starting with string and the next 3 lines
sed '/^mail/,+3s/^/#/' /etc/passwd
comment line starting with string and the next
sed '/^mail/,+1s/^/#/' /etc/passwd
comment line starting with string and the next 3 lines
sed '/^mail/,+3s/^/#/' /etc/passwd