Hi, I have the Yahoo app installed too with 3 different addresses, however I think it's just a POP mail app therefore the mail is coming one way down to your device and any changes you are are not sent back to the mail server. You'd need to use the android mail client and set up your Yahoo email as IMAP which would allow you to do what you want.
I kept these instructions from the Yahoo Mail website for my own reference, but they're what you should consider doing.
IMAP is another way you can get Yahoo! Mail on your phone. Using IMAP you can download messages with your mobile client, so you can access your mail even when not connected to the mobile Internet.
How to set up Yahoo! Mail with your mobile client using IMAP
On your phone, press < Home >, then open the Email application.
On the 'Your accounts' page, select Next to get started with setup.
Enter your full Yahoo! address and password, then select Next.
On the next screen, you can give the account a nickname and choose the name to display on your outgoing messages. Tap Done.
If prompted, supply the following information:
Incoming Server - imap.mail.yahoo.com
Outgoing Server - smtp.mail.yahoo.com
Incoming Port - 993 (requires SSL)
Outgoing Port - 465 (requires SSL)
Username: full email address (bill@yahoo.com)
Password: is the password you login to Yahoo! with
