feat: make MAIL FROM subdomain configurable via MAIL_FROM_SUBDOMAIN env var

.env.self-host.example

@@ -52,6 +52,13 @@ SES_CONFIGURATION_SET=plunk-configuration-set
 # When set, projects can choose to disable email tracking
 SES_CONFIGURATION_SET_NO_TRACKING=plunk-no-tracking-configuration-set
 
+# 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. an R2/CDN custom domain), since the
+# MAIL FROM hostname needs MX + TXT records that can't coexist with a CNAME.
+# Example: MAIL_FROM_SUBDOMAIN=emails  →  emails.<your-domain>
+# MAIL_FROM_SUBDOMAIN=
+
 # ========================================
 # OPTIONAL: OAuth Login
 # ========================================