+error codes indicate temporary errors:
+.TP 5
+.B 51
+Out of memory.
+.TP
+.B 52
+Timeout.
+.TP
+.B 53
+Write error; e.g., disk full.
+.TP
+.B 54
+Unable to read the message or envelope.
+.TP
+.B 55
+Unable to read a configuration file.
+(Not used by
+.BR qmail-queue .)
+.TP
+.B 56
+Problem making a network connection from this host.
+(Not used by
+.BR qmail-queue .)
+.TP
+.B 61
+Problem with the qmail home directory.
+.TP
+.B 62
+Problem with the queue directory.
+.TP
+.B 63
+Problem with queue/pid.
+.TP
+.B 64
+Problem with queue/mess.
+.TP
+.B 65
+Problem with queue/intd.
+.TP
+.B 66
+Problem with queue/todo.
+.TP
+.B 71
+Mail server temporarily refuses to send the message to any recipients.
+(Not used by
+.BR qmail-queue .)
+.TP
+.B 72
+Connection to mail server timed out.
+(Not used by
+.BR qmail-queue .)
+.TP
+.B 73
+Connection to mail server rejected.
+(Not used by
+.BR qmail-queue .)
+.TP
+.B 74
+Connection to mail server succeeded,
+but communication failed.
+(Not used by
+.BR qmail-queue .)
+.TP
+.B 81
+Internal bug; e.g., segmentation fault.
+.TP
+.B 91
+Envelope format error.