It sounds like the software is buggy and likely to have some compatibility issues with Windows 10.
Is there any reason your not using webmail via yahoo.com or Windows Live Mail?
The contacts you e-mail do not need to be notified, it would be entirely seamless to the recipient.
If you go to mail.yahoo.com and login with your e-mail and password ***** can then compose and read e-mails, all within the web page and without installing any software.
Have a look and let me know how you get on.
Are you told to download a program when going to the website or are you able to just use the website, there is a significant difference between using a website and a program when it comes to sending emails.
Once I know which your using I can better advise you.
In terms of e-mailing 650 contacts, you add the contacts in the TO field and click send, from a recipient perspective it makes not difference they will receive the e-mail and need not doing anything different.
You can access yahoo without downloading any files when you login there is an option for webmail.
Log back in using the link above discard any options for downloading anything and just navigate to the option for webmail, where you can send and receive e-mails from your web browser.
It sounds like we will need to do a remote session where I can securely connect to your PC and you can show me the issue.
Unfortunately the question is undervalued and as such unable to assist you, I have passed the query to customers services to refund you as requested. Since to get to the bottom of this you will require remote assistance.