Steps to reproduce:

1. Copy 600 words to the clipboard.
2. Select a user in Participants.
3. Use the "With selected users..." dropdown box to "Send a message".
4. Paste the words, and click "Send message to 1 person."
5. A popup shows "Message sent to 1 person."
6. From the messaging interface, verify that the message was never sent.

When attempting to send the same message via the messaging interface, the message gets cut at 594 words, and is sent successfully.