diff --git a/Lib/imaplib.py b/Lib/imaplib.py
index 00a17fbf36d..e2a05818fa6 100644
--- a/Lib/imaplib.py
+++ b/Lib/imaplib.py
@@ -23,6 +23,7 @@ Public functions:       Internaldate2tuple
 __version__ = "2.58"
 
 import binascii, errno, random, re, socket, subprocess, sys, time, calendar
+from io import DEFAULT_BUFFER_SIZE
 
 try:
     import ssl
@@ -1237,6 +1238,7 @@ class IMAP4_stream(IMAP4):
         self.sock = None
         self.file = None
         self.process = subprocess.Popen(self.command,
+            bufsize=DEFAULT_BUFFER_SIZE,
             stdin=subprocess.PIPE, stdout=subprocess.PIPE,
             shell=True, close_fds=True)
         self.writefile = self.process.stdin
diff --git a/Misc/ACKS b/Misc/ACKS
index b5c80592f5c..6ea3f78d76c 100644
--- a/Misc/ACKS
+++ b/Misc/ACKS
@@ -1098,6 +1098,7 @@ Richard Townsend
 Nathan Trapuzzano
 Laurence Tratt
 John Tromp
+Diane Trout
 Jason Trowbridge
 Brent Tubbs
 Anthony Tuininga
diff --git a/Misc/NEWS b/Misc/NEWS
index 5e36c4215b1..9a62a02bebe 100644
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -233,6 +233,10 @@ Core and Builtins
 Library
 -------
 
+- Issue #17443: impalib.IMAP4_stream was using the default unbuffered IO
+  in subprocess, but the imap code assumes buffered IO.  In Python2 this
+  worked by accident.  IMAP4_stream now explicitly uses buffered IO.
+
 - Issue #17476: Fixed regression relative to Python2 in undocumented pydoc
   'allmethods'; it was missing unbound methods on the class.