Problem with mail

hi , because my free hosting doesn't permit to send mail via smtp i change configuration to send mail with 'mail' , but i have this problem :
Call to undefined function CodeIgniter\Email\mail()
SYSTEMPATH/Email/Email.php at line 1762

How can i solve it ?

with my hosting i started with free hosting, then upgraded to paying. I have facility for 20 sub domains. To cut a long story short , with the sub domains and the way things are st up, i was unable to configure using the host smtp. By using PHPMailer and relaying through my own google email, i have it working that data submited by web users , submitting a form, get relayed through my google account to my yahoo account

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