For yahoo mail you have two options if you are using
yahoo email you can upgrade to yahoo mail plus to get
the full pop3 service for access to
Outlook,
Thunderbird,
Incredimail, and others
Click this link for yahoo mail pop3 settings
Or you can use free ypops fully compatible with yahoo mail and works
with Outlook Incredimail Thunderbird
and others

gmail is easier but

Incoming Mail (POP3) Server: pop.mail.yahoo.com (Use SSL, port: 995)
Outgoing Mail (SMTP) Server: smtp.mail.yahoo.com (Use SSL, port: 465, use authentication)
Account Name/Login Name: Your Yahoo! Mail ID (your email address without the “@yahoo.com”)
Email Address: Your Yahoo! Mail address (e.g.,)
Password: Your Yahoo! Mail password

If you want to use Outlook, Outlook Express, Thunderbird or other third party email clients to view your email you have 2 options.
Subscribe to mail plus:

or download and install YPOPS

then follow the directions for your version of outlook here,

If you have trouble with set up, check the YPOPS installation page for alternate set up instructions.

Click on YPOPS link on the left.
Then Click on Configuring email clients towards the bottom.
Choose the appropriate link for your mail client.

I currently utilize version 2.0.0.4 of Thunderbird
and version 0.8.3 of YPOPS (there is a more recent version of YPOPS that you need if you are Yahoo Beta Mail) and have no problems.