Hi there. My name is*****'m sorry to hear that you are having this problem. I will do my best to help you resolve the issue. My first instinct would be to do as you did, and ask them to check their spam folder. It sounds simple but you would be surprised how many people don't know how to do that. Most email programs allow you to mark a message as NOT SPAM once you've found it so future messages won't go to spam and that would be a good next step if indeed the message did go to spam.
May I ask how you are sending the messages? Do you have your own domain name and email server, or are you using gmail or yahoo or microsoft servers, or something else? Many email servers limit the number of messages that can be sent from an account per day. We might need to check that as well.
I would suggest that we begin with a simple test. For those members who report that they did not receive the email, try to send them an individual email from the same account that you use for the distribution list. If it goes through, then the problem is likely with the email server.
It would be a good idea to send it with a delivery notification requested, if your email program allows that. The rejection notice can provide important clues about the cause of the problem. If it indicates that delivery was successful, then you know that either it has to be in their SPAM folder.
Try to note whether there are any similarities in the messages that aren't delivered. For instance, are most of them from the same 2 or 3 domain names, or are most of them last in the distribution list (although that shouldn't matter). If you see any similarities at all, please let me know
Try these suggestions and let me know the results so I will know what we need to do next. Thank you!