If you use POP3 protocol and collect email using email client software (i.e. Apple’s Mail, Thunderbird, Outlook, etc.), then your email client automatically deletes these emails from the server once it collects them; they are no longer kept on the server. If you have other synced devices (such as a smartphone or tablet), it is possible you will no longer see these messages, nor will you see them when you login to your PSMailbox. In some cases, an archive manager or a storage settings could also remove emails from the server.
We recommend you use IMAP (rather than POP3) if you would like access to your mailbox from multiple devices. IMAP also allows you to mark messages as READ/UNREAD in one system (like webmail) and see such status/labels carried over when you view messages using a different email client or device.