It can be really hard to diagnose these things. At once place I worked, I know that 90% of inbound mail was spam. That gave some impression of what mail providers are dealing with. And as I have pointed out to others, the Inter-net is exactly what it says. It's not a single system, but an interconnection of different systems. Each system administrator can accept traffic or not. Given the spam situation, things that look like spam can easily get rejected. Mass mailings are one of those. 40 isn't very "mass", but it's some. That, the content and the format could be enough to trigger issues.
You could try varying one or two things. Send from a different system. In GMail, click the ellipsis at bottom right when composing, and change to plain-text mode. Send smaller numbers, perhaps in batches of 10, if possible avoiding too many going to the same recipient mail provider. None of these are changing things that you "should not" do. The idea is just to change some things that can be seen as indicators of spam. It's often a scoring system, so the cumulative effect of sending large batches of messages, with lots of links in them, in HTML (fancy) format, containing certain wordings, can all affect the chances of delivery.
