Aspects of the disclosure relate to processing external messages using a secure email relay. A computing platform may receive, from a message source server associated with a first domain, a first email message and a first set of authentication credentials. Based on validating the first set of authentication credentials, the computing platform may inject, into the first email message, a DomainKeys Identified Mail (DKIM) signature of a second domain different from the first domain, which may produce a signed message that identifies itself as originating from the second domain. Based on scanning and validating content of the signed message, the computing platform may send the signed message to a message recipient server, which may cause the message recipient server to validate the DKIM signature of the signed message and determine that the signed message passes Domain-based Message Authentication, Reporting and Conformance (DMARC) with respect to the second domain.