| mail.header.subject | Constraint checks the value against the email’s subject.

| mail.header.from | Constraint checks the value against the email’s from. This will be truthy if any value matches.

| mail.header.to | Constraint checks the value against the email’s to. This will be truthy if any value matches.

| mail.header.replyTo | Constraint checks the value against the email’s reply to. This will be truthy if any value matches.

| mail.header.cc | Constraint checks the value against the email’s cc. This will be truthy if any value matches.

| mail.header.bcc | Constraint checks the value against the email’s bcc. This will be truthy if any value matches.