user_mail_tokens is undefined
Matt -
The user_mail_tokens is a function provided by the user module (Drupal 7 core). Essentially, what your code is saying is:

If the function user_mail_tokens exists (which it should because it's a core function, however these conditional checks are awesome and important), then we are going to include an include file located at /sites/all/modules/modulename/modulename_e.inc.

As to why your code is throwing an error for user_mail_tokens that is an interesting question. I'm looking into it now. Part of me wonders if we should  just say module_exists

What is the need to check the user_mail_tokens?