Understanding Sudden Decline in Email Delivery Rate

Hi very new to all this but trying to understand what's happened to my delivery rate. I've a very small sub base (70ish) as I only started a few weeks ago but I've had a sudden decline. 3rd post - Delivered 67. Delivery Rate 100.00% 4th post - Delivered 57. Delivery Rate 76.00% 5th post - Delivered 56. Delivery Rate 74.67% I'm using a custom domain and afaik have records setup correctly. I did notice on a hotmail account I have that these posts are not even getting through to spam so something is up. Any advice is appreciated - thanks.