Ever since I can remember after using eM Client, it appears that this issue is only common with eMClient, not other third party clients like Gmail, Spark, Outlook, etc.
What happens is upon launch of eM Client, everything appears to be running normally. eM Client pulls all the smpt email accounts including Yahoo.
Usually (not always) iff one replies or composes an email with a Yahoo account, the email is sent normally. Other times, you have to shut down eM Client and refresh. However, this has been rare lately.
What’s not rare and is consistent is Yahoo SMTP in updating and retrieving Yahoo mail.
As an example, I can have Outlook Mail (web) open in my browser. I can manually delete the emails from eM Client or vice versa. The change is reflected immediately in real time. This is not the case with Yahoo where eM Client needs to shut down.
Again, the issue appears to be an issue with eM Client only, not other third party email clients. It doesn’t appear when using Mozilla Thunderbird either.