Spammers uses software to generate random email
for spamming and don't remove bounced or undelivered
mails, hence, ISPs assumed that the sender is trying to spam. 

Valid AOL, Yahoo, Hotmail addresses,
yet my emails were bounced?

Most ISPs or desktop email client, simply filters, accept your mails and delete them--you won't even know about it, until your friends or customers complain that you are ignoring their mails.
See Email Filtering

Keep in mind that AOL doesn't show the FROM: name that you provide, it only shows readers the FROM address, so make sure you are not sending with a typical spam like looking email address like this example: jonessnews_1234@myemaildomain.com
that is as good as telling recipients or the spam filters "you can delete this mail".

From: name? It is the name that you have enter into your email client ie: Outlook Express, Name: field--(start-up outlook, >Tools, >Accounts, >Properties and at the "User Information" Name: field--that's the From: name that won't show-up for AOL recipients and the FROM address? E-Mail address: field -- That's what will show-up for AOL recipients.

Many ISPs mail servers are known to block or barred a sender email domain because they may have sent or relaying large quantities of emails and/or repeatedly send emails to non-existing account holders within their domain.

Spammers are know to use software that generate random email addresses for spamming and don't bother to remove bounced or undelivered mails, hence, ISPs assumed that the sender is trying to spam by repeatedly send mails to non-existing account holders within their domain.
See Server-time-out, concurrent connection

Try using another email domain (a different sender email domain address) and avoid maintaining only AOL, Hotmail, etc. email domain addresses in one single database list, mix them together with other email domains.

Useful sites to locate new ISPs:

• www.emailaddresses.com

• www.isps.com/

• www.thelist.com/

• www.findanisp.com/

Important Notes:
If you are using your own mail server program and facing bounced, undeliverable or deleted mails by AOL, Yahoo, Hotmail, etc.
See: Mail Server FAQs