From 8ca031797f9d29854d11a00f2d5eef22faf77f1b Mon Sep 17 00:00:00 2001
From: LE MOINE Laurent <llemoine@softwaymedical.fr>
Date: Wed, 18 Jun 2025 11:01:34 +0200
Subject: [PATCH] Added support for maximum command size during XOAUTH2
 authentication

---
 src/SMTP.php | 33 ++++++++++++++++++++++++++++++---
 1 file changed, 30 insertions(+), 3 deletions(-)

diff --git a/src/SMTP.php b/src/SMTP.php
index 7226ee93..aebdc5e7 100644
--- a/src/SMTP.php
+++ b/src/SMTP.php
@@ -634,9 +634,36 @@ class SMTP
                 }
                 $oauth = $OAuth->getOauth64();
 
-                //Start authentication
-                if (!$this->sendCommand('AUTH', 'AUTH XOAUTH2 ' . $oauth, 235)) {
-                    return false;
+                /*
+                 * The maximum length for an SMTP commands is 512 bytes, according to RFC 4954 (https://datatracker.ietf.org/doc/html/rfc4954).
+                 * (In truth this seems more complex than that, but 512 bytes seems to be the stricter limit)
+                 *
+                 * Therefor, the base64-encoded OAUTH token has a maximum length of 497 : 512 - 13 (AUTH XOAUTH2) - 2 (CRLF)
+                 * If the token is longer than that, the command and the token must be sent separately
+                 */
+                if (strlen($oauth) <= 497) {
+                    //Start authentication
+                    if (!$this->sendCommand('AUTH', 'AUTH XOAUTH2 ' . $oauth, 235)) {
+                        return false;
+                    }
+                } else {
+                    // Send the command and expect a code 334
+                    if (!$this->sendCommand('AUTH', 'AUTH XOAUTH2', 334)) {
+                        return false;
+                    }
+
+                    // Send the token
+                    if (!$this->sendCommand('Oauth TOKEN', $oauth, [235, 334])) {
+                        return false;
+                    }
+
+                    // If the server answer with a code 334, send and empty line an wait for a code 235
+                    if (
+                        substr($this->last_reply, 0, 3) == 334
+                        && $this->sendCommand('AUTH End', '', 235)
+                    ) {
+                        return false;
+                    }
                 }
                 break;
             default: