diff --git a/src/sbbs3/mailsrvr.c b/src/sbbs3/mailsrvr.c
index 56ae292340a89221860640b7d584547869f51aff..d48e2572e281c5dd3b480df3290de445be5f67c0 100644
--- a/src/sbbs3/mailsrvr.c
+++ b/src/sbbs3/mailsrvr.c
@@ -2279,7 +2279,7 @@ static void smtp_thread(void* arg)
 			continue;
 		}
 		if(!stricmp(buf,"AUTH CRAM-MD5")) {
-			sprintf(challenge,"<%u.%u@%s>",socket,clock(),startup->host_name);
+			sprintf(challenge,"<%u.%lu@%s>",socket,clock(),startup->host_name);
 			/***
 			lprintf("%04d SMTP CRAM-MD5 challenge: %s"
 				,socket,challenge);