PHP Sendmail not working

I'm trying to use PHPMailer to send a simple email message from a contact form, however I'm getting errors that it is not working because PHP could not instantiate the mail function.

PHP error logs are showing the following: [Fri Oct 13 15:39:12.533834 2017] [cgi:error] [pid 27230] [client 192.186.127.194:60714] AH01215: sendmail: authentication failed (method CRAM-MD5), referer: http://cf-annual.brandhealth.ca/contact [Fri Oct 13 15:39:12.533912 2017] [cgi:error] [pid 27230] [client 192.186.127.194:60714] AH01215: sendmail: server message: 535 5.7.8 Error: authentication failed: authentication failure, referer: http://cf-annual.brandhealth.ca/contact [Fri Oct 13 15:39:12.533943 2017] [cgi:error] [pid 27230] [client 192.186.127.194:60714] AH01215: sendmail: could not send mail (account default from dummy), referer: http://cf-annual.brandhealth.ca/contact

In trying to debug this, I'm also noticing that none of my WordPress installs are able to send emails either, such as the "I forgot my password" email.