diff --git a/snappymail/v/0.0.0/app/libraries/snappymail/http/request.php b/snappymail/v/0.0.0/app/libraries/snappymail/http/request.php
index 8c1c3604a..19c8c2123 100644
--- a/snappymail/v/0.0.0/app/libraries/snappymail/http/request.php
+++ b/snappymail/v/0.0.0/app/libraries/snappymail/http/request.php
@@ -166,4 +166,29 @@ abstract class Request
 	{
 		return self::$scheme_ports[$scheme] ?? 0;
 	}
+
+	/**
+	 * Extra headers as the flat list of "Name: value" lines both transports send.
+	 *
+	 * Callers pass either form: a list of ready lines, or an associative array
+	 * such as ['Accept-Encoding' => 'gzip']. The
+	 * associative form used to reach CURLOPT_HTTPHEADER as bare values, so the
+	 * header was sent as a malformed line and dropped.
+	 *
+	 * Some callers file a complete line under its own name
+	 * (['Authorization' => 'Authorization: Basic ...'] in the socket 401
+	 * retry). Such a value is kept as it is, otherwise it would go out
+	 * as "Authorization: Authorization: Basic ...".
+	 */
+	public static function headerLines(array $headers) : array
+	{
+		$lines = array();
+		foreach ($headers as $name => $value) {
+			$value = (string) $value;
+			$lines[] = (\is_int($name) || 0 === \stripos($value, $name . ':'))
+				? $value
+				: "{$name}: {$value}";
+		}
+		return $lines;
+	}
 }
diff --git a/snappymail/v/0.0.0/app/libraries/snappymail/http/request/curl.php b/snappymail/v/0.0.0/app/libraries/snappymail/http/request/curl.php
index f25fef1c0..1c838b989 100644
--- a/snappymail/v/0.0.0/app/libraries/snappymail/http/request/curl.php
+++ b/snappymail/v/0.0.0/app/libraries/snappymail/http/request/curl.php
@@ -47,7 +47,7 @@ class CURL extends \SnappyMail\HTTP\Request
 			\curl_setopt($c, CURLOPT_CAINFO, $this->ca_bundle);
 		}
 		if ($extra_headers) {
-			\curl_setopt($c, CURLOPT_HTTPHEADER, $extra_headers);
+			\curl_setopt($c, CURLOPT_HTTPHEADER, static::headerLines($extra_headers));
 		}
 		if ($this->auth['user'] && $this->auth['type']) {
 			if ($this->auth['type'] & self::AUTH_BEARER ) {
diff --git a/snappymail/v/0.0.0/app/libraries/snappymail/http/request/socket.php b/snappymail/v/0.0.0/app/libraries/snappymail/http/request/socket.php
index 8b6ffc975..f70167427 100644
--- a/snappymail/v/0.0.0/app/libraries/snappymail/http/request/socket.php
+++ b/snappymail/v/0.0.0/app/libraries/snappymail/http/request/socket.php
@@ -43,7 +43,7 @@ class Socket extends \SnappyMail\HTTP\Request
 			$extra_headers['Authorization'] = static::$Authorization[$host];
 		}
 		if ($extra_headers) {
-			$headers = \array_merge($headers, $extra_headers);
+			$headers = \array_merge($headers, static::headerLines($extra_headers));
 		}
 		$headers = \implode("\r\n", $headers);
 		if (!\is_null($body)) {