feat: make MAIL FROM subdomain configurable via MAIL_FROM_SUBDOMAIN env var

apps/api/src/app/constants.ts

@@ -45,6 +45,12 @@ export const AWS_SES_REGION = validateEnv('AWS_SES_REGION');
 export const AWS_SES_ACCESS_KEY_ID = validateEnv('AWS_SES_ACCESS_KEY_ID');
 export const AWS_SES_SECRET_ACCESS_KEY = validateEnv('AWS_SES_SECRET_ACCESS_KEY');
 
+// Custom MAIL FROM subdomain used to construct `<subdomain>.<your-domain>`
+// when a domain is added. Defaults to `plunk`. Override when `plunk.<your-domain>`
+// is already used for something else (e.g. a CDN), since the MAIL FROM hostname
+// needs MX + TXT records that can't coexist with a CNAME.
+export const MAIL_FROM_SUBDOMAIN = validateEnv('MAIL_FROM_SUBDOMAIN', '').trim() || 'plunk';
+
 // Email Processing Rate Limit (optional override)
 // If not set, will automatically fetch from AWS SES account quota
 // Set this to override AWS quota (useful for setting lower limits or testing)

apps/api/src/controllers/Config.ts

@@ -8,6 +8,7 @@ import {
   GITHUB_OAUTH_ENABLED,
   GOOGLE_OAUTH_ENABLED,
   LANDING_URI,
+  MAIL_FROM_SUBDOMAIN,
   NODE_ENV,
   S3_ENABLED,
   SMTP_DOMAIN,
@@ -62,6 +63,7 @@ export class Config {
       },
       aws: {
         sesRegion: AWS_SES_REGION,
+        mailFromSubdomain: MAIL_FROM_SUBDOMAIN,
       },
     });
   }

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 ?? [];

apps/web/src/components/DomainsSettings.tsx

@@ -335,6 +335,8 @@ export function DomainsSettings({projectId}: DomainsSettingsProps) {
             <div className="space-y-4">
               {domains.map(domain => {
                 const status = getDomainStatus(domain);
+                const mailFromSubdomain = config?.aws?.mailFromSubdomain ?? 'plunk';
+                const mailFromHost = `${mailFromSubdomain}.${domain.domain}`;
                 return (
                   <div key={domain.id} className="border border-neutral-200 rounded-lg p-4">
                     <div className="flex items-center justify-between mb-3">
@@ -494,8 +496,8 @@ export function DomainsSettings({projectId}: DomainsSettingsProps) {
                                   </Badge>
                                 </div>
                                 <p className="text-xs text-neutral-600 mb-2">
-                                  Set up a custom MAIL FROM domain (plunk.{domain.domain}) to improve deliverability and
-                                  handle bounces/complaints.
+                                  Set up a custom MAIL FROM domain ({mailFromHost}) to improve deliverability and handle
+                                  bounces/complaints.
                                 </p>
 
                                 <div className="overflow-x-auto">
@@ -522,16 +524,16 @@ export function DomainsSettings({projectId}: DomainsSettingsProps) {
                                         <td className="py-3 px-3">
                                           <div className="flex items-center gap-2">
                                             <code className="text-xs font-mono text-neutral-700 break-all flex-1">
-                                              plunk.{domain.domain}
+                                              {mailFromHost}
                                             </code>
                                             <Button
                                               variant="ghost"
                                               size="sm"
-                                              onClick={() => handleCopyToken(`plunk.${domain.domain}`, 3000)}
+                                              onClick={() => handleCopyToken(mailFromHost, 3000)}
                                               className="shrink-0 h-6 w-6 p-0 overflow-hidden"
                                             >
                                               <AnimatedCopyIcon
-                                                isCopied={copiedToken === `plunk.${domain.domain}-3000`}
+                                                isCopied={copiedToken === `${mailFromHost}-3000`}
                                               />
                                             </Button>
                                           </div>
@@ -571,16 +573,16 @@ export function DomainsSettings({projectId}: DomainsSettingsProps) {
                                         <td className="py-3 px-3">
                                           <div className="flex items-center gap-2">
                                             <code className="text-xs font-mono text-neutral-700 break-all flex-1">
-                                              plunk.{domain.domain}
+                                              {mailFromHost}
                                             </code>
                                             <Button
                                               variant="ghost"
                                               size="sm"
-                                              onClick={() => handleCopyToken(`plunk.${domain.domain}`, 3001)}
+                                              onClick={() => handleCopyToken(mailFromHost, 3001)}
                                               className="shrink-0 h-6 w-6 p-0 overflow-hidden"
                                             >
                                               <AnimatedCopyIcon
-                                                isCopied={copiedToken === `plunk.${domain.domain}-3001`}
+                                                isCopied={copiedToken === `${mailFromHost}-3001`}
                                               />
                                             </Button>
                                           </div>

apps/web/src/lib/hooks/useConfig.ts

@@ -21,6 +21,7 @@ export interface ConfigResponse {
   };
   aws: {
     sesRegion: string;
+    mailFromSubdomain: string;
   };
 }