I regret to inform you that it is an ongoing issue with Yahoo mail and there is no solution for this yet.
But there is a workaround if you want to use only Yahoo email.
Until they fix this issue, type your emails into "Notepad" or "Microsoft word" and then copy it and paste it all directly into Yahoo mail and click Send.
OR I suggest you to use other provide like Gmail. As per my experience, Gmail is fast, better and secure and there is no such issue there.
We are Justanswer team, an independent tech support service provider. My job is provide you with honest and truthful information.
I hope you won't blame me for this by leaving negative feedback.
If you have any other query regarding this issue, please do not hesitate to let me know.
Please take a second to rate my service by clicking stars at the top of the screen then submit. Bonus/tip is highly appreciated. Thank you, ***** *****