yitz: send.xtra.co.nz was redirected from Yahoo to the SMX platform on 2nd Feb. So was mail delivery. It has nothing to do with whether your account shows on their checker as migrated or not, that will happen in batches in the coming month(s) with one weeks notice.
I never said it was anything to do with any type of checker, and this problem also has nothing to do with stored email, it is solely related to sending emails via a third party email address. The fact is that people have been unable to send email via the send.xtra email server since around the 2/2/2017, if using a third party authenticated address, and they never provided a fix or even temporary fix. They also have never told customers that they switched the send.xtra server, they have just said that there are technical problems with yahoo. The only thing they do have on their status page is about old versions of outlook no longer being supported, but this problem isn't related.
SENDING OR RECEIVING EMAILS USING THIRD PARTY EMAIL CLIENTS
Some customers may be experiencing problems receiving Yahoo! Xtra mail when using third party email clients such as outlook. Yahoo! Mail can still be accessed using webmail at xtramail.co.nz. Investigations have shown that this is affecting customers on older email clients and upgrading to a more secure modern client resolves the issues. If you are experiencing these issues and are on an older email client we recommend upgrading to a more secure modern client.