Is there a way you can send an email to a distribution list? This is not working as only the individual email addresses are working fine. Is there any setting that i am missing?
I dealt with this issue and found out it had to do with mail server restrictions put in place by Sys Admins at my workplace.
A lot of places have rules in place regarding which users or servers can access distribution lists, so the first thing I would do is ask your SA team if such restrictions exist.
Another thing to try (as another answer stated) is setting default content type to text/html - assuming above restrictions aren’t the cause.