Sending and receiving email are two entirely separate systems. If you can receive email OK, then you have the POP or IMAP server configured correctly, meaning the port number, login credentials, transport, and the name of the server.
So, your problem is with the SMTP server. There are often more problems with this, partly because of the potential for abuse. The original specification did not even have any kind of authentication, but along came spammers, and associated problems.
Anyway, there are now several ways to send mail, assuming the service even allows it - some will force you to use your ISP for outgoing mail, although this is getting rare.
I don't use Evolution, so I can't tell you step by step how to configure it, but you need to look for either SMTP or outgoing mail server, and enter the information for Yahoo, which according to one search result on Yahoo is:
Outgoing Server: smtp.mail.yahoo.com
Outgoing Server (SMTP) port: 465
Also, somewhere you need to tell it the server
requires an encrypted connection (SSL)
Evolution may require the server and port number to be specified together, as "smtp.mail.yahoo.com:465"