That’s because the hostname of your server is mail.whatever
Anyways, if you want to use another template:
sudo su -
mkdir /usr/local/hestia/data/templates/email
cp /usr/local/hestia/install/common/templates/email/examples/account_ready.html /usr/local/hestia/data/templates/email/
Now edit this file and change it to fit your needs:
/usr/local/hestia/data/templates/email/account_ready.html
If you want to use another language (you should translate it by your own), copy the file but add the country code, example for Spain:
/usr/local/hestia/data/templates/email/account_ready.es.html