A computer-implemented method, the method being performed by a secure relay computing platform and comprising the steps of: receiving (203, 305), via a communication interface of the secure relay computing platform, from a message source server associated with a first domain, a first email message and a first set of authentication credentials; validating (204, 310) the first set of authentication credentials; based on validating (204, 310) the first set of authentication credentials, injecting (205, 315) into the first email message, a DomainKeys Identified Mail, DKIM, signature of a second domain different from the first domain, wherein injecting (205, 315) the DKIM signature of the second domain into the first email message produces a first signed message that identifies itself as originating from the second domain; scanning (206, 320) content of the first signed message, wherein scanning (206, 320) the content of the first signed message produces first scan results; validating (207, 325) the first scan results; and based on validating (207, 325) the first scan results, sending (208, 330), via the communication interface, to a message recipient server, the first signed message, wherein sending (208, 330) the first signed message to the message recipient server causes the message recipient server to validate the DKIM signature of the first signed message and determine, based on validating the DKIM signature of the first signed message, that the first signed message passes Domain-based Message Authentication, Reporting and Conformance, DMARC, with respect to the second domain; wherein the first domain is a domain name corresponding to a first entity; wherein the second domain is a domain name corresponding to a second entity different from the first entity; and wherein the secure relay computing platform is configured by a relay client computing ecosystem linked to the second entity using the second domain. receiving (203, 305), via a communication interface of the secure relay computing platform, from a message source server associated with a first domain, a first email message and a first set of authentication credentials; validating (204, 310) the first set of authentication credentials; based on validating (204, 310) the first set of authentication credentials, injecting (205, 315) into the first email message, a DomainKeys Identified Mail, DKIM, signature of a second domain different from the first domain, wherein injecting (205, 315) the DKIM signature of the second domain into the first email message produces a first signed message that identifies itself as originating from the second domain; scanning (206, 320) content of the first signed message, wherein scanning (206, 320) the content of the first signed message produces first scan results; validating (207, 325) the first scan results; and based on validating (207, 325) the first scan results, sending (208, 330), via the communication interface, to a message recipient server, the first signed message, wherein sending (208, 330) the first signed message to the message recipient server causes the message recipient server to validate the DKIM signature of the first signed message and determine, based on validating the DKIM signature of the first signed message, that the first signed message passes Domain-based Message Authentication, Reporting and Conformance, DMARC, with respect to the second domain; wherein the first domain is a domain name corresponding to a first entity; wherein the second domain is a domain name corresponding to a second entity different from the first entity; and wherein the secure relay computing platform is configured by a relay client computing ecosystem linked to the second entity using the second domain.