feat: make MAIL FROM subdomain configurable via MAIL_FROM_SUBDOMAIN env var

apps/api/src/services/SESService.ts

@@ -6,6 +6,7 @@ import {
   AWS_SES_REGION,
   AWS_SES_SECRET_ACCESS_KEY,
   DASHBOARD_URI,
+  MAIL_FROM_SUBDOMAIN,
   SES_CONFIGURATION_SET,
   SES_CONFIGURATION_SET_NO_TRACKING,
   TRACKING_TOGGLE_ENABLED,
@@ -250,10 +251,13 @@ export const verifyDomain = async (domain: string): Promise<string[]> => {
   // Verify DKIM for the domain
   const DKIM = await ses.verifyDomainDkim({Domain: domain});
 
-  // Set custom MAIL FROM domain (plunk.yourdomain.com)
+  // Set custom MAIL FROM domain. The subdomain defaults to `plunk` and can be
+  // overridden via the MAIL_FROM_SUBDOMAIN env var — useful when `plunk.<domain>`
+  // is already in use for something else (e.g., a CNAME to a CDN), since the
+  // MAIL FROM subdomain needs MX + TXT records that conflict with a CNAME.
   await ses.setIdentityMailFromDomain({
     Identity: domain,
-    MailFromDomain: `plunk.${domain}`,
+    MailFromDomain: `${MAIL_FROM_SUBDOMAIN}.${domain}`,
   });
 
   return DKIM.DkimTokens ?? [];