Mattermost is a well-known open-source messaging platform used by teams, but mattermost.veevadev.com appears to be an unaffiliated instance with some worrying gaps. For a site that likely handles user accounts and messages, the complete lack of visible ownership is the biggest concern. Legitimate SaaS providers almost always list their company name, address, and registration details publicly — this one hides all of that behind a private WHOIS registration.
There's also no record of this site in the Wayback Machine, and the domain's robots.txt file blocks all search engine crawlers. That doesn't automatically mean something bad is going on, but it makes it harder to verify claims or find user experiences. If you're considering using this instance for team communication, start by asking the person who invited you for direct proof of who operates it. Without a known company behind it, you're trusting an anonymous operator with potentially sensitive conversations.
Is mattermost.veevadev.com a scam? There's not enough evidence to say it is, but there isn't enough to say it's safe either. The signals point to a setup that avoids scrutiny rather than inviting it.