diff --git a/aiosmtpd/smtp.py b/aiosmtpd/smtp.py
index 45d5d92bc..3ccc80eda 100644
--- a/aiosmtpd/smtp.py
+++ b/aiosmtpd/smtp.py
@@ -583,10 +583,23 @@ def smtp_DATA(self, arg):
         yield from self.push('354 End data with <CR><LF>.<CR><LF>')
         data = []
         num_bytes = 0
+        unfinished_line = []
         size_exceeded = False
         while True:
-            line = yield from self._reader.readline()
-            if line == b'.\r\n':
+            # Since eof_received cancels this coroutine,
+            # readuntil() can never raise asyncio.IncompleteReadError.
+            try:
+                line = yield from self._reader.readuntil()
+                assert line.endswith(b'\n')
+            except asyncio.LimitOverrunError as e:
+                # The line exceeds StreamReader's buffer.
+                # XXX Should we respond with 500 Line too long
+                # and set size_exceeded = True here?
+                # How is an SMTP implementation supposed to react?
+                line = yield from self._reader.read(e.consumed)
+                assert not line.endswith(b'\n')
+            # A lone dot in a line signals the end of DATA.
+            if not unfinished_line and line == b'.\r\n':
                 if data:
                     data[-1] = data[-1].rstrip(b'\r\n')
                 break
@@ -597,10 +610,18 @@ def smtp_DATA(self, arg):
                 size_exceeded = True
                 yield from self.push('552 Error: Too much mail data')
             if not size_exceeded:
-                data.append(line)
+                if line.endswith(b'\n'):
+                    data.append(EMPTYBYTES.join(unfinished_line) + line)
+                    del unfinished_line[:]
+                else:
+                    unfinished_line.append(line)
+            elif line.endswith(b'\n'):
+                del unfinished_line[:]
         if size_exceeded:
             self._set_post_data_state()
             return
+        # If unfinished_line is non-empty, then the connection was closed.
+        assert not unfinished_line
         # Remove extraneous carriage returns and de-transparency
         # according to RFC 5321, Section 4.5.2.
         for i in range(len(data)):