PHP mailers are, as the name suggests, tools intended to send e-mails using php commands. CMSs like WordPress, Joomla or Magento have builtin functions to send e-mails. However, malware creators have their own specially crafted scripts to send messages. Such scripts can be used for sending spam, phishing e-mails, flooding mailboxes or stealing and sending sensitive data back to the malware creators.
Any vulnerable PHP based website. Outdated software or compromised passwords can act as an infection vector.
Inspect your site's files looking for mail() functions on files that you don't recognize. Also you can sign up with us and let our team remove the malware for you.