Sent new password works for some addresses, not others

[ksolito]

I have a user who's trying to get their password resent and they're not receiving anything. I tried it and got nothing. I contacted my web host's tech support and it worked for them. I then went in to the admin and changed the email address to my hotmail accound, exited admin and again submitted the send password request. I received the password email at my hotmail account almost immediately. I tried it a second time with the same result. I then changed my email address back to my initial email account, submitted the send password request but never received it. To help rule out anti-spam interferrance, I shut down my email client and used my webmail which only marks spam as such, but still never received it.

 

Does anybody have even the slightest idea why it only seems to fail with certain email addresses?