Do you access Gmail messages on two or more computers via POP clients like Outlook or Thunderbird. The problem (or advantage) with POP access is that once an email message gets downloaded onto one computer, it is skipped by other email clients.
To explain that further - let’s say you have configured both your personal laptop and work computer to download emails from your Gmail account via POP access. Now if the emails are downloaded to the laptop first, they wont be available on your work computer (and vice-versa).
While this is a time-saving feature, your emails are split across different computers. To trick your POP client into downloading Gmail emails that are already fetched by other clients, just use the recent mode in Gmail.
This trick will also work on custom domains if you are using Google Apps with Gmail.