Ubercart

Yes - it is version 1.6

Regarding the email address, are these email addresses the same as what the domain is for your site? In other words, they purchase a file from www.example.com - do the receipts get emailed to user@example.com as well? In your testing or otherwise?

I have found that with our server we sometimes have trouble sending to the same domain name, because the mail system needs to be configured (internal DNS and all that) in the server itself in order to properly send an email to its own domain.

If the users are not getting emails period, at different domains, then I assume it's something with your mail service or possibly other smtp settings. You're probably better off calling a tech support person to figure out exactly what the problem is. (Also, in my experience, the email receipts get caught in spam folders and missed by the recipients.)

Regarding the "can only download once" issue, is it related to this topic: http://www.ubercart.org/forum/support/8302/sql_error_file_download - have you tried downloading the file once, reloading the page, and then downloading again? We have a limit set for our users - 20 I believe - and we don't have this issue... we have a separate, similar issue, which involves the use of the "cache" database table - which stores uc_file download data - preventing users from downloading files. They receive the "you have attempted to download an incorrect URL too many times" error.

Let me know if any of this is sounding familiar.