> For viva.com's engineering team, in case this reaches you: add a Message-ID header to your outgoing transactional emails.

Don't know what they're using for sending emails, but that's something that should be handled by their email service provider, unless they're hosting their own email servers.

Interestingly the MX record of via.com points to Google, but their verification emails could come from anywhere of course. The IP address in the log is also a Google IP, although that could also be the receiving IP.