Add configurable SMTP message size limit

Replace hardcoded 20MB SMTP message size limit with configurable option.
The new configuration allows users to adjust the maximum message size
accepted by the built-in SMTP server to match their mail server limits.

Changes:
- Add DEFAULT_MAX_SMTP_MESSAGE_SIZE constant (25MB default)
- Implement EENGINE_MAX_SMTP_MESSAGE_SIZE environment variable
- Add smtp.maxMessageSize configuration option
- Update help.txt with new --smtp.maxMessageSize option
- Add configuration example to default.toml

Configuration options (in priority order):
1. Environment variable: EENGINE_MAX_SMTP_MESSAGE_SIZE=35M
2. Config file: [smtp] maxMessageSize = "35M"
3. Command line: --smtp.maxMessageSize=35M
4. Default: 25MB

This addresses issues where users with larger mail server limits
(e.g., Exchange Online 35MB) were unable to send messages through
EmailEngine SMTP server due to the previous hardcoded 20MB limit.

Author: andris9

config/default.toml

@@ -27,6 +27,7 @@ port = 2525
 host = "127.0.0.1"
 secret = ""        # client password, if not set allows any password
 proxy = false      # Set to true if using HAProxy with send-proxy option
+#maxMessageSize = "25M" # maximum message size accepted by SMTP server (default: 25MB)
 
 [dbs]
 # redis connection

help.txt

@@ -41,11 +41,14 @@ Options:
   --queues.submit           Submit concurrency count        [number] [default: 1]
 
   Submission SMTP server:
-  --smtp.enabled     Is SMTP submission server enabled [boolean] [default: false]
-  --smtp.secret      SMTP password for all accounts                      [string]
-  --smtp.host        Host to bind to              [string] [default: "127.0.0.1"]
-  --smtp.port        Port to bind to                     [number] [default: 2525]
-  --smtp.proxy       Is the SMTP server hehind HAProxy [boolean] [default: false]
+  --smtp.enabled         Is SMTP submission server enabled
+                                                   [boolean] [default: false]
+  --smtp.secret          SMTP password for all accounts                  [string]
+  --smtp.host            Host to bind to          [string] [default: "127.0.0.1"]
+  --smtp.port            Port to bind to                 [number] [default: 2525]
+  --smtp.proxy           Is the SMTP server hehind HAProxy
+                                                   [boolean] [default: false]
+  --smtp.maxMessageSize  Max message size        [number/string] [default: "25M"]
 
 "emailengine encrypt" additional options:
   --decrypt  Old secret for decrypting (can be issued multiple times)    [string]

lib/consts.js

@@ -110,6 +110,7 @@ module.exports = {
 
     DEFAULT_EENGINE_TIMEOUT: 10 * 1000,
     DEFAULT_MAX_ATTACHMENT_SIZE: 5 * 1024 * 1024,
+    DEFAULT_MAX_SMTP_MESSAGE_SIZE: 25 * 1024 * 1024, // 25MB
 
     // IF IMAP authentications fail more than provided value, then disable IMAP for the account
     DEFAULT_MAX_IMAP_AUTH_FAILURE_TIME: 3 * 24 * 3600 * 1000, // 3 days

workers/smtp.js

@@ -6,7 +6,7 @@ const packageData = require('../package.json');
 const config = require('wild-config');
 const logger = require('../lib/logger');
 
-const { getDuration, emitChangeEvent, readEnvValue, matchIp, threadStats, loadTlsConfig } = require('../lib/tools');
+const { getDuration, emitChangeEvent, readEnvValue, matchIp, threadStats, loadTlsConfig, getByteSize } = require('../lib/tools');
 
 const Bugsnag = require('@bugsnag/js');
 if (readEnvValue('BUGSNAG_API_KEY')) {
@@ -54,13 +54,13 @@ config.smtp = config.smtp || {
 
 config.service = config.service || {};
 
-const MAX_SIZE = 20 * 1024 * 1024;
+const { REDIS_PREFIX, DEFAULT_MAX_SMTP_MESSAGE_SIZE } = require('../lib/consts');
+
 const DEFAULT_EENGINE_TIMEOUT = 10 * 1000;
 
+const MAX_SMTP_MESSAGE_SIZE = getByteSize(readEnvValue('EENGINE_MAX_SMTP_MESSAGE_SIZE') || config.smtp.maxMessageSize) || DEFAULT_MAX_SMTP_MESSAGE_SIZE;
 const EENGINE_TIMEOUT = getDuration(readEnvValue('EENGINE_TIMEOUT') || config.service.commandTimeout) || DEFAULT_EENGINE_TIMEOUT;
 
-const { REDIS_PREFIX } = require('../lib/consts');
-
 const ACCOUNT_CACHE = new WeakMap();
 
 let callQueue = new Map();
@@ -281,7 +281,7 @@ async function init() {
         logger: smtpLogger,
         disableReverseLookup: true,
         banner: 'EmailEngine MSA',
-        size: MAX_SIZE,
+        size: MAX_SMTP_MESSAGE_SIZE,
         useProxy: await settings.get('smtpServerProxy')
     };