diff --git a/src/agents/smtp/smtpc.c b/src/agents/smtp/smtpc.c
index a35d892..4974632 100755
--- a/src/agents/smtp/smtpc.c
+++ b/src/agents/smtp/smtpc.c
@@ -39,6 +39,8 @@
 #include "mailauth.h"
 
 #include "smtpc.h"
+
+#define SMTP_OUTBOUND_WORKER_STACK_SIZE (512 * 1024)
 #include "capabilities.h"
 #include "protocol.h"
 #include "proxy.h"
@@ -1965,7 +1967,15 @@ SMTPAgentServer(void *ignored)
 
     /* Listen for incoming queue items.  Call ProcessEntry with a
      * SMTPAgentClient allocated for each incoming queue entry. */
-    SMTPAgent.OutgoingThreadPool = BongoThreadPoolNew(AGENT_NAME " Clients", BONGO_QUEUE_AGENT_DEFAULT_STACK_SIZE, 0, 1, minSleep);
+    /*
+     * DNSSEC validation in libunbound and TLS policy processing require
+     * substantially more stack than the legacy queue-agent default.  The
+     * XPL pthread wrapper doubles the requested value, yielding a 1 MiB
+     * stack while retaining a guard page for this SMTP delivery worker.
+     */
+    SMTPAgent.OutgoingThreadPool = BongoThreadPoolNew(
+        AGENT_NAME " Clients", SMTP_OUTBOUND_WORKER_STACK_SIZE,
+        0, 1, minSleep);
     //SMTPAgent.OutgoingThreadPool = BongoThreadPoolNew(AGENT_NAME " Clients", BONGO_QUEUE_AGENT_DEFAULT_STACK_SIZE, minThreads, maxThreads, minSleep);
     SMTPAgent.nmapOutgoing = BongoQueueConnectionInit(&SMTPAgent.agent, Q_OUTGOING);
     BongoQueueAgentListenWithClientPool(&SMTPAgent.agent,