One thing I have seen mentioned many times on forums is people asking how they can send and receive mail under the following circumstances:-

1). If you have purchased a domain name and wish to send mail using it, but still connect via your ISP.
2). If you have recently moved to another ISP but still wish to send mail using your old email address.

In circumstance 1). This is quite easy because you normally will have mail forwarded to your existing ISP via the control panel with whom you have purchased your domain name, so you simply need to set up a new account within your mail client to send mail using your ISPs mail servers.

In the case of 2). You will still need a separate account within OE which can still check your mail. The reason for this is because you will need to connect on your old ISP servers to collect mail and therefore need your old username details to connect to the old ISP. You will also need an account with the details of your current ISPs SMTP server from which you can send mail through since your username and password for the two are likely to be different.

To do this from within OE

Tools >>
Accounts >>
Mail Tab >>
Click the "Add" Button >>
Select "Mail".

This will start the Internet Connection Wizard

• Enter a Display Name - This will be the name that gets shown in the contact field when you send mail to other people.
  eg. Kitz
  
• Enter the e-mail address of the account which you wish the email should appear to have come from
  eg. me@emailaddress.co.uk
• Enter the incoming (POP3) mail server.
  This should be the server address of your ISP.
  eg mail.plus.net
• Enter the outgoing (SMTP) mail server.
  This has to be the server address of the ISP who you are currently with
  eg relay.plus.net
• Enter the username your ISP has given you
  
• Enter the password for your ISP
• Click Next >> Finish
  

You should now see that you have created a new mail account - however it may display in the account pane with the POP3 server of the ISP for the account you have just created.
You can easily change this:

From within the mail window highlight the new mail account >>
Click the properties button >>
Under the "General" Tab >>
Change the name in the first field to what ever you like.

Finally it is very important to remember if you use this account, it will check mail on your existing ISP and collect any mail from there. If you have other mail accounts for your ISP setup this may be inconvenient.
Therefore you will need to ensure that you only use this account to send mail. To stop OE checking mail on your current ISP when you use this account

Uncheck the tick box at the bottom marked
"Include this account when receiving mail or synchronizing".