AT&T email woes break Apple Mail

After experiencing multiple failures when sending email from an AT&T IMAP account, AT&T Email Support advised yesterday that there are unresolved issues between AT&T and Yahoo that are causing these failures. AT&T staff assert that Yahoo has been asked for information that has not been provided, sounds like a stalemate.

Although intermittent issues have been experienced since August 2017 with iOS devices, the problem has now also happened on my desktop running MacOS 10.11.6. When attempting to send an email, immediately, the message displayed is "Cannot sent mail. A copy has been placed in your Outbox. The recipient "XXXXXX" was rejected by the server.

After extensive efforts with Apple Support and AT&T Email support, the explanation reported above was given by AT&T yesterday, 3/9/18.

The workaround is to use AT&T email online with a browser.

It is unknown whether AT&T and Yahoo will resolve their issues.

Has anyone else experienced these issues? What resolutions were found?