Are they listed in the Email Delivery History tab of your Subscription in that group? YES
In which section are they listed?
If in the Most Recent Successful Delivery section that's the end of the story as far as Groups.io is concerned. Your email service accepted delivery, what they did with the message from there you'll have to discuss with them.
If in the Most Recent Unsuccessful Attempt section that's indicitave of greylisting. The affected message(s) should eventually either be delivered or be rejected ("bounced") depending on how Groups.io's retries interact with your email service's greylisting procedures.
I have had similar problems with Yahoo groups not delivering all my group's postings.
This indicates that your email service isn't very keen on delivering email list traffic to you. I recommend choosing a better service.
Many years ago (in the DSL days) I switched from Verizon to a cable service as my ISP. In part because of the price/speed benefit, but also in part because Verizon would swallow some of my Y!Group messages and I couldn't get any satisfaction from their technical support. That's my rant.
Shal
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