POP3 is a version of POP, or Post Office Protocol - a protocol designed to allow single user computers to retrieve electronic mail from a POP server. POP3 allows you to download messages from Crawler servers onto your computer so you can access them with a mail client such as MS Outlook or Thunderbird, even when you are not online.
SMTP, or Simple Mail Transfer Protocol is a protocol used to transfer electronic mail between computers. The setting of SMTP server allows you to send messages through your Crawler email account without the need to be connected to the Internet all the time. You can type your messages using a mail client such as MS Outlook and send them later when you go online.
When you download your messages to your computer using POP3 access, a copy will remain on the server and when you access your webmail, they will appear as being unread. However, once you download messages using POP3 access, they cannot be downloaded the same way again since they will be marked as already having been downloaded.