I can’t quite figure out what’s going on here with a few users on one domain vs Gmail.
Any time they send to someone from this one domain (the only one on this server that has issues) to a Gmail address it bounces with a “security warning”. No explanation or clue what the issue was. With or without attachment, it complains and rejects. The Gmail page it sends you to doesn’t help much at all, since it lists file extensions it doesn’t like (yet it also fails with none) and this, for messages without attachments:

Sometimes messages are blocked when you don’t include any attachments.This can happen when you include content, images, or links that might share viruses.

Cool.
Since Google in general and Gmail specifically basically have no help contacts, no “list of naughty senders” to see if we’re on it and so on what do we do now? The server IP isn’t on any of the hundreds of RBLs, spam checkers and so on. Good rep everywhere…just this one issue with a few senders on one server.
Frustrating. Any pointer at all would be appreciated.