1. What does the "From" look like on the received email? Does it show the sender's address, or some groups.io sender?
2. What happens if the mail bounces?
3. If you can only mail someone twice in 24 hours, that's not much of a DM capability. If responses also go through groups.io, maybe it can reset that counter when there's a reply.
4. If the recipient sends a message back like "STOP" or clicks a link, can it stop that sender from sending direct mails to that recipient? If this feature isn't there, the recipient might start marking the messages as spam, and you know where that leads.
5. I'm sure someone will want the ability to send a message to multiple people. Is that a good feature, or should they just create a group for that sort of thing?
Just typing things off the top of my head.