Take a look at what Matt suggests. Also look if your email changed or the list has changed in some way.
Have your subject lines changed?
Do an AB test to see what happens as well.
Check your delivered % too.
Thank you guys. Matt, that release info is good to know, thanks. I will take a look at other things that probably caused the decline. We have been sending that newsletter for years and that one month we seemed to dip and not recover. I'm sure that there are other issues but I was asked to check the tech side of Marketo.
Thanks for the help!