Coinciding with the most recent security update (Android version 9, security patch level September 5, 2018, Baseband version g8998-00253-1805232234, Build number PPR2.180905.005, my Pixel 2 has ceased sending MMS messages, irregardless of the app used to send them. Google Messages returns an error stating "Message not sent: invalid destination address".
The messages used to be able to send. I can still send regular SMS, and MMS messages are received (albeit delayed), but this error means that I can no longer send messages in group chats, or attach photos to texts.
What can I do?