If you suddenly noticed that mail() functions works very slowly and causes long page loads, here is quick solution.
By default php mail() functions works via SendMail service which requires having correct hostname at your server.
You can check it by typing hostname
in SSH terminal.
You probably will see something like ubuntu-16gb-nb1-1 or centos-4gb-cl1 etc.
But you must have correct hostname there. Type this command and it will set correct hostname.
After getting correct hostname add newly added one to /etc/hosts.
Open /etc/hosts with
command and you will see the line like
127.0.1.1 ubuntu-16gb-nb1-1 ubuntu-16gb-nb1-1
Comment or remove that line and add this line instead
127.0.0.1 localhost.localdomain localhost yourwebsite.com
Save it by clicking CTRL+O and that’s all. Test mail() function again, it will work as fast as rocket.