All your recipients need to put you in the "white list" in their email software, app, etc.  All email apps have that setting, but it is called by different names.  In Yahoo email, the setting is in Settings -> Filters.  If your recipients don't know how to set this up, you need to teach them.  They can also find your email in the Spam folder and mark it as "Not Spam."

I understand what you have said. Before using the Bulk Email with a real Customer List, I used it to send to 3 emails services I have accounts at. The email send to my personal Yahoo account from my business domain account was flagged as spam by Yahoo, not by the recipient. I used mail-tester.com to examine my email for likely being flagged as spam. It passed with flying colors. SpamAssassin likes my email. I have a DKIM record and SPF record.

Is the email able to reach the destination at all?  If it is, the recipient can set a filter to always let the sender's email pass through, as I described in my post.

When Yahoo marks an email as spam, there are two possible outcomes:

(1)  The email will NOT reach the recipient; Yahoo will simply kill the email.  The recipient won't get anything.  Yahoo does this to frequent senders of mass spam.

(2)  The email WILL reach the recipient, albeit most likely in the recipient's spam folder.

If it's (1), there is nothing you can do; the email is gone because Yahoo kills it.  But if it's (2), the recipient can always white-list the sender with a filter as I described.

You have no control of whether (1) or (2) will occur.  Yes, sometimes legitimate senders are mistaken as spammers.  For instance, if you send tons of email in a short amount of time (the exact amount varies), Yahoo or any email service will flag you as a spammer, and you will remain flagged until a period of time (which also varies, usually hours) has passed.

In my old job, I had set my auto-email system (made with Access) send only 10 emails every 10 minutes (60 emails per hour).  Any higher frequency would get us flagged, and we wouldn't be able to send for hours.

When (1) happens, the sender may not even get a return failure notice, as if the email was never sent, which is the intended effect.

Thank you for your insight. It is very valuable. I modified the HTML code and the problem Yahoo now likes my email. I will adjust my Send Delay to 60 seconds. I am restricting my bulk email to 250 per day to keep the ISP happy. This is a great community. I hope to get good enough to be able to help others.

You're welcome.  Also, different email servers having different configurations, and likely different time restrictions.  Some may let you send 250 times per day, but some fewer, or more.  When you send, you go through TWO systems: the SMTP server at your end, and the SMTP server at your recipient's end.  When you send to people who use AOL, Apple email, Hotmail, Mail.com, or even private domains like 599cd.com, you go through all those different servers (plus your own) and you are subject to all their restrictions.  Naturally, you need to set your schedule based on the most restrictive server.  But you often have no idea what that is.  So you'll have to keep experimenting with different schedules to find out.  If most of your recipients use Yahoo Mail, it makes your job easier since you have fewer server restrictions to deal with.

As I said, you still have to remind your recipients to check their spam folders and white-list you.  Even if your email passes through all the servers during the Internet transit, it may still end up being flagged as spam by the email app on the user's device, which you have no control over.