Qmail is one of the oldest mailing systems of this world. It is also the pioneer of the mailing system, Most of the modern mailing system was built keeping
Below command will show Max allowed message size:
cat /var/qmail/control/databytes
You can change it by below command:
echo “10485760” > /var/qmail/control/databytes
Above command will set the max message size to 10 MB.