See official documentation (link here).

Other gotchas and caveats
* Sending Email
Do not use the existing task/event trigged Mail module infrastructure for sending up signup/password reset emails.  Because the system can be configured to send emails to a text file, or silently mirror emails to multiple recipients, it's not secure and reliable.  Also, if this module is upgraded at any point in the future module developers have to update their modules to match, which may cause problems with missing email in the interim.