Mailing list messages not delivered to all

[Erica C.]

Hi,
We've been using ezmlm lists since 2006 and they have worked very well for our group. Increasingly lately, some messages are not getting through to everone. Recent messages were reported missing by subscribers with Verizon, AOL, Gmail, and Yahoo.
I have suggested that list members create filters. Do people have other suggestions and/or explanations?
Thanks!
Erica

[Erica C.]

Following up with additional information. The site associated with the mailing list doesn't have a security certificate. Would that make a difference in Gmail etc. calling our messages spam?

Thanks for any information.
Erica