diff --git a/bbs/readline.c b/bbs/readline.c
index 61cd03a..7efb70d 100644
--- a/bbs/readline.c
+++ b/bbs/readline.c
@@ -135,7 +135,7 @@ ssize_t bbs_readline(int fd, struct readline_data *restrict rldata, const char *
 #endif
 		if (rldata->left - 1 < 2) {
 			bbs_warning("Buffer (size %lu) has been exhausted\n", rldata->len); /* The using application needs to allocate a larger buffer */
-			return -1;
+			return -3;
 		}
 		res = bbs_poll_read(fd, timeout, rldata->pos, (size_t) rldata->left - 1); /* Subtract 1 for NUL */
 		if (res <= 0) {
diff --git a/include/readline.h b/include/readline.h
index 03349d0..30d9357 100644
--- a/include/readline.h
+++ b/include/readline.h
@@ -63,10 +63,13 @@ int readline_bytes_available(struct readline_data *restrict rldata, int process)
  * \param rldata Previously initialized using bbs_readline_init
  * \param delim A delimiter (can be multiple characters). CR LF is typical for most network applications.
  * \param timeout Timeout in ms for any call to poll()
- * \retval -2 on failure, -1 if read or poll returns 0, and number of bytes read in first input chunk otherwise, not including the delimiter.
- *         A return value of 0 means that only the delimiter was read.
- *         If a positive value is returned, the caller can read the input in the buffer passed to bbs_readline_init. It will be null terminated after the first input chunk,
- *         not including the delimiter.
+ * \retval -3 on buffer exhaustion
+ * \retval -2 if read or poll returned -1
+ * \retval -1 if read or poll returned 0
+ * \retval 0 Only the delimiter was read.
+ * \return Number of bytes read in first input chunk otherwise, not including the delimiter.
+ *         Caller can read input in the buffer passed to bbs_readline_init.
+ *         It will be null terminated after the first input chunk, not including the delimiter.
  * \note The actual number of bytes read may be greater than the number of bytes returned. These bytes will be returned in subsequent calls to this function.
  */
 ssize_t bbs_readline(int fd, struct readline_data *restrict rldata, const char *restrict delim, int timeout);
diff --git a/nets/net_smtp.c b/nets/net_smtp.c
index 326d1a2..3487bc1 100644
--- a/nets/net_smtp.c
+++ b/nets/net_smtp.c
@@ -2364,6 +2364,17 @@ static int handle_data(struct smtp_session *smtp, char *s, struct readline_data
 		if (res < 0) {
 			bbs_delete_file(template);
 			fclose(fp);
+			if (res == -3) {
+				/* Buffer was exhausted.
+				 * RFC 5322 2.1.1 says lines MUST be no longer than 998 characters.
+				 * This is the sending mail user agent's resonsibility, not the responsibility of any MTA.
+				 * Some user agents don't conform to this requirement, and some MTAs will happily relay them anyways.
+				 * In practice, most messages that violate this seem to be spam anyways,
+				 * so we have no reason to tolerate noncompliant messages.
+				 * However, we need to reject it properly with the right error message before disconnecting. */
+				smtp_reply_nostatus(smtp, 550, "Maximum line length exceeded");
+				smtp->failures += 3; /* Semantically, this is a bad client. However, we're just going to disconnect now anyways, so this doesn't really matter. */
+			}
 			return -1;
 		}
 		s = rldata->buf;
diff --git a/tests/test_smtp_mta.c b/tests/test_smtp_mta.c
index db69fbc..86daea9 100644
--- a/tests/test_smtp_mta.c
+++ b/tests/test_smtp_mta.c
@@ -175,6 +175,29 @@ static int run(void)
 	SWRITE(clientfd, "." ENDL); /* EOM */
 	CLIENT_EXPECT(clientfd, "554"); /* Mail loop detected */
 
+#define CHAR_31 "abc def ghi jkl mno prs tuv wxy"
+#define CHAR_248 CHAR_31 CHAR_31 CHAR_31 CHAR_31 CHAR_31 CHAR_31 CHAR_31 CHAR_31
+#define CHAR_992 CHAR_248 CHAR_248 CHAR_248 CHAR_248
+
+	/* Ensure messages with lines over 1,000 characters are rejected */
+	SWRITE(clientfd, "RSET" ENDL);
+	CLIENT_EXPECT(clientfd, "250");
+	SWRITE(clientfd, "EHLO " TEST_EXTERNAL_DOMAIN ENDL);
+	CLIENT_EXPECT_EVENTUALLY(clientfd, "250 ");
+	SWRITE(clientfd, "MAIL FROM:<" TEST_EMAIL_EXTERNAL ">\r\n");
+	CLIENT_EXPECT(clientfd, "250");
+	SWRITE(clientfd, "RCPT TO:<" TEST_EMAIL ">\r\n");
+	CLIENT_EXPECT(clientfd, "250");
+	SWRITE(clientfd, "DATA\r\n");
+	CLIENT_EXPECT(clientfd, "354");
+	SWRITE(clientfd, "Date: Thu, 21 May 1998 05:33:29 -0700" ENDL);
+	SWRITE(clientfd, ENDL);
+	SWRITE(clientfd, "Test" ENDL);
+	SWRITE(clientfd, CHAR_992 ENDL); /* This is okay */
+	SWRITE(clientfd, CHAR_992 CHAR_31 ENDL); /* This is not okay */
+	SWRITE(clientfd, "." ENDL); /* EOM */
+	CLIENT_EXPECT(clientfd, "550"); /* Mail loop detected */
+
 	/* Test pregreeting */
 	close(clientfd);
 	clientfd = test_make_socket(25);