I'm apologize if you are one of them.
I've done some research and now understand what happened. Late in the evening of April 4, without any warning in advance, Yahoo silently made a change in their DMARC authentication protocol that it doesn't allow Yahoo users to send emails not from their servers. Since my email is from Yahoo but I use external email service provider MailChimp, this is a problem and a lot of my recent emails didn't go though.
Yahoo says they made this change because of people with bad intentions who would send phishing emails and they would appear like they are legitimate but they are fraud. Since Yahoo doesn't have a way to confirm the real sender's identity, for now it's experimenting with blocking all incoming email with Yahoo addresses which go through external servers.
Many innocent bloggers worldwide like me (and their subscribers) were affected by this new change. Not only many emails bounced back, but often, because of the bounced emails, many people were removed from their subscription lists (if this is your case, please re-subscribe again at www.organduo.lt).
It seems that a person is no longer important to Yahoo. They thought that this would fix the fraudulent emails and the hacking problem and obviously it works. But to completely destroy legitimate email lists and small businesses - I think it's way too harsh (I hope they will reconsider this experiment and switch back to normal). Otherwise they will lose a lot of people because of this.
If you are interested in how and why it's happening, you can read more on this here, here and here.
I'm working on fixing it right now - changing my email address to another email service in every of my free or paid lesson email (more than 750 emails total).
I'm sorry if this has affected you in any way. If you feel that you have stopped receiving your lessons, please let me know and I'll send them to you manually.
I hope to fix this problem as fast as possible. Thank you for your patience.