How can I let people order with same e-mail if they can't log in

I have a huge re-occurring problem with repeat customers saying the password recovery e-mail never arrives, (It always works for me when I test it) Then if they try to place an order with the express checkout or make a new account it blocks them because their e-mail is already in the system.

I just ordered from a website where I couldn't log in, the password recovery e-mails were never arriving, but it simply let me make a new account with the same e-mail, so it was no big deal.

How can I get zen-cart to let someone use a repeat e-mail? That would greatly reduce how many problems I'm having with repeat customers.

I would first determine if thew emails were/are generated

In your admin > config > email options > Email Archiving Active? > set to true

Install this plugin so that you can see them in your admin
https://www.zen-cart.com/downloads.php?do=file&id=101

If they are generated - it may be that your customers are not looking in their spam box or have you whitelisted

You might also want to check with http://mxtoolbox.com/blacklists.aspx to see if you are on a blacklist. AOL, sbcglobal, att, etc will simply bit-bucket emails from blacklisted servers without telling either the sender or the receiver.