MX issue? Only some members of a group receive emails...

[spin-up]

I've been experiencing issues for a few days now where emails are not being correctly routed to the members of a group.

We have a group with the address administration@domain.ext, and only one user (who was added to test this) is receiving emails.

The other 3 are not. I've already tried removing and re-adding a user, but nothing changes.

I notice this icon next to the various members (including the user who is receiving emails correctly).

I've checked the DNS and see these values.

All personal emails are working fine.