You may also log in to your groups.io account and check tab "Your Activity" for entries like "Email to ... is bouncing via system". This indicates your mail provider is rejecting the email.
I ran into this after adding a daily digest email for the 5th groups.io group: randomly one of the five emails bounced - obviously the provider's spam filter kicked in.
My solution was changing my groups.io destination email address to a different one, hosted by a provider with less strict spam rules.
73,
Arno DL6EE
