diff --git a/hsarback/src/de/hsadmin/mods/dom/DomainModuleImpl.java b/hsarback/src/de/hsadmin/mods/dom/DomainModuleImpl.java
index c8d1d60..9459d4c 100644
--- a/hsarback/src/de/hsadmin/mods/dom/DomainModuleImpl.java
+++ b/hsarback/src/de/hsadmin/mods/dom/DomainModuleImpl.java
@@ -230,6 +230,13 @@ public class DomainModuleImpl extends AbstractModuleImpl {
 			if ("letsencrypt".equals(opt.getName()) && updatedDom.getServeraliases().contains("*")) {
 				throw new HSAdminException("invalid domain option: " + opt.getName() + " for wildcard subdomain");
 			}
+			if ("backupmxforexternalmx".equals(opt.getName())) {
+				final Query query = em.createQuery("SELECT e FROM EMailAddress e WHERE e.domain = :domname");
+				query.setParameter("domname", updatedDom.getName());
+				if (!query.getResultList().isEmpty()) {
+					throw new HSAdminException("domain option backupmaxforexternalmx may not be activated when email addresses exist");
+				}
+			}			
 		}
 		needsWriteAccessOn(oldDom, "update");
 		return super.update(existingEntity);
diff --git a/hsarback/src/de/hsadmin/mods/email/EMailAddressModuleImpl.java b/hsarback/src/de/hsadmin/mods/email/EMailAddressModuleImpl.java
index 1933357..4be83c4 100644
--- a/hsarback/src/de/hsadmin/mods/email/EMailAddressModuleImpl.java
+++ b/hsarback/src/de/hsadmin/mods/email/EMailAddressModuleImpl.java
@@ -12,6 +12,7 @@ import de.hsadmin.core.model.HSAdminException;
 import de.hsadmin.core.model.Transaction;
 import de.hsadmin.hostsharing.BasePacType;
 import de.hsadmin.mods.dom.Domain;
+import de.hsadmin.mods.dom.DomainOption;
 import de.hsadmin.mods.user.UnixUser;
 
 public class EMailAddressModuleImpl extends AbstractModuleImpl {
@@ -66,7 +67,12 @@ public class EMailAddressModuleImpl extends AbstractModuleImpl {
 		qEmailAddresses.setParameter("pacId", dom.getUser().getPac().getId());		
 		if (qEmailAliases.getResultList().size() + qEmailAddresses.getResultList().size() >= EMAIL_PER_MULTI_OPTION * dom.getUser().getPac().getQuantityByComponentName("MULTI")) {
 			throw new HSAdminException("included email addresses/aliases exceeded");
-		}					
+		}		
+		for (DomainOption opt : adr.getDomain().getDomainoptions()) {
+			if ("backupmxforexternalmx".equals(opt.getName())) {
+				throw new HSAdminException("email addresses may not be added when domain option backumxforexternalmx is active");
+			}
+		}
 		return super.add(newEntity);
 	}