Mail appears in my email client that didn't show up in MailWasher Pro

Either you have not disabled auto checking for mail in your email client and emails are being downloaded to your email client automatically, or mail has arrived in the time you have spent checking and classifying your mail in MailWasher Pro.

To disable auto email checking, see How to switch off automatic email checking.

Otherwise, once you have finished previewing and sorting your mail in MailWasher Pro, click Check Mail again to ensure no other mail has arrived in the interim.