Does BackupMX have spam filtering?
Unlike a FakeMX server which will reject the message with an SMTP 451 Error. Greylisting will tell a recipient to retry in 5 minutes, but only the first time the recipient is seen.
Most spam e-mail will try a server and if told to retry simple drops the message and moves on to the next victim. Whereas legitimate e-mail will try again and when Greylisting sees the address more than once it remembers the address so it only ever gets grey listed once.
BackupMX uses greylisting to help fight against spam and ensure the servers are doing what they are designed to best, which is service your legitimate emails and act as a SMTP Backup (SMTP Secondary) to your primary mail server.