Going into Settings > Accounts and Sync and changing the status of Background Data (if it is on, switch it off, and if it is off, switch it on.

If that does not allow you to send, in the Email app, tap Menu > More > Account Settings > Outgoing Settings and ensure that the Outgoing Server Settings read as follows:

smtp.mail.yahoo.com
SSL
465

Of course, also check Require Sign In and provide your Yahoo! Mail credentials. You can also try adding and/or removing the User Name suffix as well (i.e. @yahoo.com)

This is the screen that will confirm if you will be able to successfully send E-Mail or not from within the native Android OS Email app, in that if you receive the message:

Setup could not finish
Cannot safely connect to server. ( java.security.cert.CertPathValidatorException: Trust anchor for the certification path not found.)

then your E-Mail will not successfully send from the native Android OS Email app. However, if upon applying the settings in Menu > More > Account Settings > Outgoing Settings, the settings simply apply, then you will have a small window where you WILL be able to send E-Mail from the native Android OS Email app (be forewarned, this window lasted 2-3 messages for me, upon which point the error then returned.)

I’ve also had luck getting these settings to stick through simultaneously opening the native Android OS web browser and logging on to mail.yahoo.com through there. For some reason, accessing my E-Mail through the web browser followed by then trying to apply the settings in the native Android OS Email app (Menu > More > Account Settings > Outgoing Settings) permitted another small window in which I could send E-Mail from the native Android OS Email app.

My plan of attack through all of this is to read E-Mail using my native Android OS Email app (the sorting options are far richer than the web browser version AND the official Yahoo! Mail app) and to compose E-Mail using the web browser version.

Note that I am currently a Yahoo! Mail Classic user (and will remain one until Yahoo! forces me to change)

It Worked for me (OUTLOOK 2010)

do the following

File Menu (Office Menu)
Add Account
Manually Configure …..
Internet Email
Your Name (allkhanan)
Email Address (youremail@yahoo.com)
Select imap instead pop3
incoming server = imap.mail.yahoo.com
outgoing server = smtp.mail.yahoo.com
user name = allkhanan
password= your yahoo password

click on more setting and do the following
1- advanced tab – incoming server 995
enable this server require …..
2- out going server 993 SSL

now go to other tab and do the following
enable both
my outgoing server (smpt) require …..
use same setting as my ….

and hopefully it will work

Enjoy the free browsing and wish peace for all the mankind.