Incoming mail server generally uses either the POP or IMAP protocol. The addresses generally look something like pop.isp.com or imap.isp.com.
Your outbound server for sending your messages is still from your ISP, but it usually has smtp. in front of it, because it uses the smtp mail protocol. You could take a guess, such as smtp://isp.com.
Although guessing might work, usually the best thing to do when configuring a mail client is to refer to your mail provider's instructions - often little tweaks are needed to make it work, such as changing the port on which mail is sent. So I would suggest visiting your mail provider's website, or Googling its name and "client configuration" and see if anything turns up.
Hope this helps
John A
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For me its like:
smtp.ispname.com
pop.ispname.com
Some ISPS expect you to use your own SMTP server
jbennet
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