Topic
Per Dev, the customer's email address was placed on the Sendgrid bounce list as their server on January 17th.
(550 Rejected by header-based Anti-Spoofing policy: ttps://community.mimecast.com/docs/DOC-1369#550).
Dev has removed them from the bounce list now. However, it’s quite possible this will happen again.
This article explains what happens:https://www.twilio.com/docs/sendgrid/glossary/drops