diff --git a/source/java/org/alfresco/repo/tenant/MultiTServiceImpl.java b/source/java/org/alfresco/repo/tenant/MultiTServiceImpl.java
index fe71d4418b..fc92ebeafb 100644
--- a/source/java/org/alfresco/repo/tenant/MultiTServiceImpl.java
+++ b/source/java/org/alfresco/repo/tenant/MultiTServiceImpl.java
@@ -401,9 +401,23 @@ public class MultiTServiceImpl implements TenantService
         if (name != null)
         {
             int idx = name.lastIndexOf(SEPARATOR);
-            if (idx != -1)
+            if (idx > 0 && (idx < (name.length() - 1)))
             {
-                return name.substring(0, idx);
+                String domainPart = getTenantDomain(name.substring(idx + 1));
+                String baseName = name.substring(0, idx);
+
+                // MNT-11766 fix, check whether tenant domain actually exists
+                Tenant tenant = getTenant(domainPart);
+                if (tenant == null)
+                {
+                    // tenant domain doesn't exists but we are allowed to create non-tenant users with name like admin@test
+                    // no base name can be resolved for such users -> return original name
+                    return name;
+                }
+                else
+                {
+                    return baseName;
+                }        
             }
         }
         return name;