Hi I am new to Mattermost and Graylog from what I could gather Mattermost should be Slack compatible If I am mistaken kindly correct me.
If I setup a notification in Graylog 4.0.6 with the Slack Notification using a Mattermost URL I get
Please specify a valid webhook url
I believe there is a regex that only works for slack formatted URL’s so users don’t make a mistake but it blocks Mattermost from working though
http://mattermost.domain.com.na/hooks/gd4ihtkpnjnwbbnns5dj8fceha
http://hooks.slack.com/service/xxx/xxx/xxx
If you install Release 3.1.0 · graylog-labs/graylog-plugin-slack · GitHub then it is possible to use Mattermost with a Legacy Slack Alarm Callback
Following these instructions
https://docs.stackhero.io/en/Graylog/Alerting
The coming problem
Legacy alarm callbacks are deprecated and will be removed in Graylog 4.1. Please switch to the new notification types as soon as possible!
Then the current hack solution will be dead. 
Developer gods please hear me (and others) plea!
Thank you for a very cool product!