Since the transition to Yahoo mail yesterday, I can receive emails on my iPhone, but I cannot send.

Yes I understood. The last line indicates to change "Outlook" to Apple Mail" when choosing the app at Yahoo. 

Thank you. Yes; I went back and read that. However, I did follow the instructions, but to no avail. 

Hi Msgdiaz7, as an Apple Mail App user myself, I am sorry you're having issues sending emails. Did you change the setting with the existing email account you already had set up or did you create a new? To create a new go to Settings > Mail > Accounts > Add Account > Yahoo! > Sign in with Cox Email and Password. I transitioned 3 emails and did this process with no issues. There is not much assistance we can provide outside of these links, https://www.cox.com/residential/support/access-and-log-in-to-yahoo-mail-after-transition.html . If you need additional assistance, you can reach out to Yahoo directly for assistance. The support is included for the first 90 days. 

This has been a disaster.  It took me more than an hour to get the yahoo to work through Mac Mail on my desktop.  I could then receive emails, but not send, on both my iPhone and iPad.  The solution by gbru allowed my iPad to send, at the cost of losing all the emails that were on my iPad when the account was deleted.  It did not work on my iPhone.  This sucks.  Cox should have made this a lot easier.  "If you like your doctor, you can keep your doctor...."

This solution worked perfectly for me. Thank you. Hadn't realized I kept the other account. IOS seems to fall back to that for sending. 

When I start to delete my old account, I get a message that I interpret as all my old emails will be deleted. Can someone confirm that the old emails are kept?

Yes all old emails and saved items are kept because yahoo migrated everything over already to their platform. all good. 

Thank you for your reply.  This is the warning I receive when starting to delete the old cox account from my Mac Mail: "The account’s setup information, mailboxes, and all messages will be deleted permanently from your computer. Copies of messages on the mail server, if they exist, will not be affected."  When I did this on my iPad, I lost the messages that were on my iPad but no longer on the server.  I can't afford to lose all the emails that are in folders on my Mac, and (I suspect) no longer on the server.  Can you confirm that your old messages are still available?