Upgrade to smtp-server 3.18.4

Author: mattherman

package-lock.json

@@ -70,7 +70,7 @@
     "proper-lockfile": "4.1.2",
     "safe-regex": "^2.1.1",
     "safe-stable-stringify": "2.5.0",
-    "smtp-server": "3.18.1",
+    "smtp-server": "3.18.4",
     "winston": "3.17.0",
     "xpath": "0.0.34",
     "yargs": "17.7.2"