diff --git a/Lib/smtplib.py b/Lib/smtplib.py
--- a/Lib/smtplib.py
+++ b/Lib/smtplib.py
@@ -57,6 +57,7 @@
 SMTP_PORT = 25
 SMTP_SSL_PORT = 465
 CRLF="\r\n"
+_MAXLINE = 8192 # more than 8 times larger than RFC 821, 4.5.3
 
 OLDSTYLE_AUTH = re.compile(r"auth=(.*)", re.I)
 
@@ -128,6 +129,10 @@
     combination provided.
     """
 
+class SMTPTooLongError(SMTPResponseException):
+    """Line too long."""
+
+
 def quoteaddr(addr):
     """Quote a subset of the email addresses defined by RFC 821.
 
@@ -170,13 +175,17 @@
         def __init__(self, sslobj):
             self.sslobj = sslobj
 
-        def readline(self):
+        def readline(self, size=-1):
+            if size < 0:
+                size = None
             str = ""
             chr = None
             while chr != "\n":
                 chr = self.sslobj.read(1)
                 if not chr: break
                 str += chr
+                if size is not None and len(str) == size:
+                    break
             return str
 
         def close(self):
@@ -334,11 +343,13 @@
         if self.file is None:
             self.file = self.sock.makefile('rb')
         while 1:
-            line = self.file.readline()
+            line = self.file.readline(_MAXLINE + 1)
             if line == '':
                 self.close()
                 raise SMTPServerDisconnected("Connection unexpectedly closed")
-            if self.debuglevel > 0: print>>stderr, 'reply:', repr(line)
+            if self.debuglevel > 0: print >>stderr, 'reply:', repr(line)
+            if len(line) > _MAXLINE:
+                raise SMTPTooLongError(500, "Line too long.")
             resp.append(line[4:].strip())
             code=line[:3]
             # Check that the error code is syntactically correct.