Strengthening Email Ecosystem: Outlook’s New Requirements for High‐Volume Senders

As a member of a church organization, I am the registrar for a seminar and have 108 couples registered.  I tried to send update notices to my couples, most of whom have two addresses so 216 messages.  I encountered several bounced notices due to this error.  I am using my normal Yahoo address that I have had for almost 30 years.  I rarely send over 20 email messages in a day, but occasionally send out group replies.  Now, in rare cases over the years, I have had to send out more messages in a day, but never even up to 500 in one day.  My fingers would give out!  What am I doing to encounter these errors and how do I correct the situation so my people get their notices?