Why Don't Some Employees / Users Receive a Password Reset?

Summary

For some, clicking "Forgot Password" for EWS or Manager Site doesn't get them a reset password?

Content

Are there any settings that I'm not aware of that impact whether clicking Forgot Password for either EWS or Manager site works or doesn't work?  Even though I test this periodically I still have employees and users who click and then tell me they never get a password reset back.  Some I suppose get trapped in junk due to the e-mail address used to send it to them (invalidaddress@ . . .).

First timers seem to have this frequently.

Thanks in advance for any advice, Harold