Sun Java System Messaging Server 6.3 Administration Guide

26.3.5 Timeouts on Incoming SMTP connections

Timeouts on incoming SMTP connections are most often related to system resources and their allocation. The following techniques can be used to identify the causes of timeouts on incoming SMTP connections:

ProcedureTo Identify the Causes of Timeouts on Incoming SMTP Connections

  1. Check how many simultaneous incoming SMTP connections you allow. This is controlled by the MAX_PROCS and MAX_CONNS Dispatcher settings for the SMTP service; the number of simultaneous connections allowed is MAX_PROCS*MAX_CONNS. If you can afford the system resources, consider raising this number if it is too low for your usage.

  2. Another technique you can use is to open a TELNET session.

    In the following example, the user connects to port 25. Once connected, 220 banner is returned. For example:

    telnet 25
    Connected to
    Escape character is ’^]’.
    220 --Server ESMTP (Sun Java System Messaging Server 6.1 
    (built May  7 2001))

    If you are connected and receive a 220 banner, but additional commands (like ehlo and mail from) do not illicit a response, then you should run imsimta test -rewrite to ensure that the configuration is correct.

  3. If the response time of the 220 banner is slow, and if running the pstack command on the SMTP server shows the following iii_res* functions (these functions indicate that a name resolution lookup is being performed):

    febe2c04 iii_res_send (fb7f4564, 28, fb7f4de0, 400, fb7f458c, fb7f4564) + 
    42c febdfdcc iii_res_query (0, fb7f4564, c, fb7f4de0, 400, 7f) + 254

    then it is likely that the host has to do reverse name resolution lookups, even on a common pair like localhost/ To prevent such a performance slowdown, you should reorder your host’s lookups in the /etc/nsswitch.conf file. To do so, change the following line in the /etc/nsswitch.conf file from:

    hosts: dns nis [NOTFOUND=return] files


    hosts: files dns nis [NOTFOUND=return]

    Making this change in the /etc/nsswitch.conf file can improve performance as fewer SMTP servers have to handle messages instead of multiple SMTP servers having to perform unnecessary lookups.

  4. You can also put the slave_debug keyword on the channels handling incoming SMTP over TCP/IP mail, usually tcp_local and tcp_intranet. After doing so, review the most recent tcp_local_slave.log-uniqueid files to identify any particular characteristics of the messages that time out. For example, if incoming messages with large numbers of recipients are timing out, consider using the expandlimit keyword on the channel.

    Remember that if your system is overloaded and overextended, timeouts will be difficult to avoid entirely.