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POP3 I/O Error

JoAnn Nagel
Occasional Contributor

POP3 I/O Error

I have ixUW-IMAP A.18.00-2007f.001 loaded on my HP-UX 11.23 system in order to use POP3.  It works great until mail has been idle for awhile, it seems.  It will work fine all day, but in the morning, I begin seeing the following errors in my mail.log:


Mar 30 09:05:20 masd078x sendmail[22489]: q2UD5C622489:, size=5958, class="0", nrcpts=1, msgid=<>, bodytype=7BIT, proto=ESMTP, daemon=MTA,[]

Mar 30 09:05:20 masd078x sendmail[22490]: q2UD5C622489:, delay=00:00:08, xdelay=00:00:00, mailer=local, pri=31704, dsn=4.0.0, stat=I/O error


In researching, I found that POP3 has a 10 minute autologout timer by default, so I'm guessing that's what is happening, but I can't find how to prevent this from happening or where to increase/turn off the autologout.  Any help would be greatly appreciated.

Honored Contributor

Re: POP3 I/O Error

These messages are from the sendmail daemon. It is the "Mail Transfer Agent" (MTA): it talks to other mail servers, and manages the local mail spool files (normally in /var/mail in HP-UX). The only time your email client contacts Sendmail directly is when it is sending an outgoing message.


UW-IMAP, on the other hand, only gives you access to your email inbox (and possibly other email folders stored on the server) using a variety of protocols, i.e. POP3, IMAP and possibly their SSL/TLS secured varieties.


UW-IMAP and Sendmail have a standard mechanism for coordinating their access to your email inbox, and Sendmail has its own local spool area so it would not have to reject an email because you are currently accessing your inbox.


The log messages suggest a mail was incoming from to user sdtest on your server: (IP address was delivering it to your server. But your server could not accept the incoming message at all: it had to report a temporary failure code (dsn=4.0.0) to the mail server.


Even if the inbox file of the sdtest user (/var/mail/sdtest) was busy, Sendmail would have accepted the email, stored it temporarily into /var/spool/mqueue, and then delivered it to the user's inbox when the inbox was accessible again.


This suggests something might be wrong with your local mail spool directory (/var/spool/mqueue): perhaps the /var filesystem is full or the disk is starting to fail?


If your HP-UX system has a static, public IP address and its SMTP port (TCP/25) is open to Internet for the purpose of receiving email globally, you will soon be receiving automated attempts to use your server as a mail relay for the purposes of sending junk email and email-based malware. If you have not configured your server with adequate anti-relay protections, your server may accidentally have become an "open relay":


In that situation, your /var/spool/mqueue will soon fill up to 100% by junk emails addressed to remote, possibly non-existent addresses, and the error messages resulting from your server failing to deliver the messages sent to it.


If this has already happened, *STOP* Sendmail on your system as soon as possible. Otherwise your server and possibly your entire organization will be recognized as a spam relay, and may be blacklisted. This is likely to cause problems in your legitimate outgoing email communications. Next step will probably be to inspect your mail spool ("mailq" should be an useful command here) and remove the spam infestation from /var/spool/mqueue. If you can see that there are no legitimate emails in the mailq listing, you can simply delete everything in /var/spool/mqueue as Sendmail is already stopped.


Before restarting Sendmail on your system, you should learn about the spam and malware issues of today's Internet email and apply the necessary configuration changes to protect your server from misuse. If your site has another mail server, you should talk with its administrator: there are ways to provide full email connectivity to your server without subjecting it directly to the raw Internet torrent of spam and malware.


If you're the only person at your site with any email server experience, googling for "sendmail anti-relay antispam" might be a good way to start. Be advised that HP-UX includes a file that is different from the "vanilla" Sendmail default configuration: before making any configuration changes, make sure they are applicable to HP-UX Sendmail specifically.