A common mistake many users who keep copies of their email in their POP3 mailbox make when setting up a new email account is forgetting to change the setting to leave mail on the server, so Outlook downloads and deletes it from the server.
If this happens to you, you can restore the deleted mail to the server only if you have IMAP access to your mailbox.
When your email account supports IMAP, you can restore mail to the server in a few simple steps:
Gmail: configure the account for IMAP in your GMAIL settings then setup the email account in Outlook. Gear icon, Mail settings, Forwarding and POP/IMAP link)
Yahoo: use imap-ssl.mail.yahoo.com for the IMAP server name. In Outlook 2007 and 2010 you need to create the account manually.
Hotmail: although Hotmail does not support IMAP, you can use the Outlook Hotmail Connector to upload mail to the server.
Note that both Gmail and Hotmail make it difficult for Outlook to delete mail from the server. If Outlook deleted downloaded email from either account, check your settings in the account online and configure it to archive mail downloaded using POP3.
If your email account does not support IMAP, you can't easily restore mail to the server unless it can POP email from other servers. In this case, you could upload the mail to another server then POP it back to your original email account. If this is not possible and you need online access to the mail you downloaded, consider using Gmail, Hotmail, or Yahoo to store it online for you.
If your account can collect mail from POP accounts (like Gmail, Yahoo and Hotmail do), you can upload the mail using IMAP then your account can collect it using POP3. Before using this method, verify your POP3 account can collect mail from the IMAP server - GMail and Hotmail require SSL for POP3 services and many accounts do not use SSL when "popping" accounts.