See below copied from WordPress Codex - http://codex.wordpress.org/FAQ_Troubleshooting#E-mailed_passwords_are_not_being_received
Ensure Proper Return Address is Used
By default, the WordPress mailer fills in the From: field with firstname.lastname@example.org and the From: name as WordPress. This is fine if this is a valid e-mail address.
For example, if your real e-mail is email@example.com, your host should pass the email on for delivery. It will probably send your mail as long as yourdomain.com is setup to send and receive mail, even if wordpress is not a valid mail box.
But if you set you real email as the From: address and it’s something like firstname.lastname@example.org, the mail may not send because gmail.com is not a domain handled by the mail server.