X-headers store custom fields in email messages to support routing, analytics, and automation.
X-Headers are custom email headers (often starting with X-) used to attach metadata to a message. Teams use them to identify which campaign sent the email, which automation rule created it, or which customer record it maps to. While some legacy headers are being phased out, custom headers are still useful for internal automation and debugging. In an email automation platform, X-headers can help with threading, attribution, and safe parsing. The key is to keep them consistent, documented, and not overly sensitive.