feat: restrict smtp sender address (#3637)

* fix: check if sender address is custom domain

* fix: check if sender address is custom domain

* fix: check if sender address is custom domain

Co-authored-by: Livio Amstutz <livio.a@gmail.com>

internal/domain/policy_domain.go

@@ -7,7 +7,8 @@ import (
 type DomainPolicy struct {
 	models.ObjectRoot
 
-	UserLoginMustBeDomain bool
-	ValidateOrgDomains    bool
-	Default               bool
+	UserLoginMustBeDomain                  bool
+	ValidateOrgDomains                     bool
+	SMTPSenderAddressMatchesInstanceDomain bool
+	Default                                bool
 }