Emails from "Subscribe by Email" are not Making it To Everyone on The List

Hi:

I have you plugin installed at cwla.org. An email is generated and sent weekly with the titles and excerpts of new posts. I received notice from a few folks that they were no longer receiving emails. I checked the email logs and there were no errors reported and I have been receiving the emails regularly. I'm at a loss for what may be causing this. Any tips on troubleshooting this?