MAILADDR=(mail1@example.com, mail2@example.com, mail3@example.com)
for i in "${MAILADDR[@]}"
    do
         echo "Mail test..." | mail -s "Mail test subject..." $i
    done

echo"/home/user/information-for-any-user.txt"  | mail -s "Mail test subject..." $i ??

mail -s "My subject" $(cat /this/is/the/external/flat/file.txt) < /home/user/information-for-any-user.txt

echo "Mail test..." | mail -s "Mail test subject..." $(< address.file)

mapfile -t addr < address.file
echo "Mail test..." | mail -s "Mail test subject..." "${addr[@]}"