Posts Tagged ‘Express’

How to exclude for a particular IP address in Qmail Mail server install / Fix to Thunderbird mail sent error (Exploitable Server See: error

Tuesday, November 1st, 2011

In the office, some of my colleagues has started receiving error messages, while trying to send mail with Thunderbird and Outlook Express
The exact error they handed to me reads like this:

An error occured while sending mail. The mail server responded: Exploitable Server See: Please check the message recipient

Here is also a screenshot, I’ve been sent via Skype with the error poping up on a Thunderbird installed on Windows host.

Typing the url lead me to to a page saying that the IP address of the mail client which is trying to send mail is blacklisted . This is not strange at all condireng that many of the office computers are running Windows and periodically get infected with Viruses and Spyware which does sent a number of Unsolicated Mail (SPAM).

The record for the IP seems to be an old one, since at the present time the office network was reported to be clear from malicious SMTP traffic.

The error disallowing the mail clients to send from the office continued for already 3 days, so something had to be done.

We asked the ISP to change the blacklisted IP address of xx.xx.xx.xx , to another one but they said it will take some time and they can’t do it in a good timely matter, hence to make mail sending work again with POP3 and IMAP protocols from the blacklisted IPs I had to set in the Qmail install to not check the xx.xx.xx.xx IP against mail blacklisting databases.

On qmail install disabling an IP check in RBLSMTPD is done through editting /etc/tcp.smtp and following recreate of /etc/tcp.smtp.cdb – red by qmailctl script start.
The exact line I put in the end of /etc/tcp.smtp to disable the RBLSMTPD check is:


Further on to recreate /etc/tcp.smtp.cdb and reload the new cdb db records:

qmail:~# qmailctl cdb
qmail:~# qmailctl restart

Onwards, the IP blacklist issue was solved and all office computers from xx.xx.xx.xx succeeded in sending mails via SMTP.

How to fix “imapd-ssl: Maximum connection limit reached for” imapd-ssl error

Saturday, May 28th, 2011

One of the mail server clients is running into issues with secured SSL IMAP connections ( he has to use a multiple email accounts on the same computer).
I was informed that part of the email addresses are working correctly, however the newly created ones were failing to authenticate even though all the Outlook Express email configuration was correct as well as the username and password typed in were a real existing credentials on the vpopmail server.

Initially I thought, something is wrong with his newly configured emails but it seems all the settings were perfectly correct!

After a lot of wondering what might be wrong I was dumb enough not to check my imap log files.

After checking in my /var/log/mail.log which is the default log file I’ve configured for vpopmail and some of my qmail server services, I found the following error repeating again and again:

imapd-ssl: Maximum connection limit reached for" imapd-ssl error

where was the email user computer IP address.

This issues was caused by one of my configuration settings in the imapd-ssl and imap config file:


In /usr/lib/courier-imap/etc/imapd there is a config segment called
Maximum number of connections to accept from the same IP address

Right below this commented text is the variable:


As you can see it seems I used some very low value for the maximum number of connections from one and the same IP address.
I suppose my logic to set such a low value was my desire to protect the IMAP server from Denial of Service attacks, however 4 is really too low and causes problem, thus to solve the mail connection issues for the user I raised the MAXPERIP value to 50:


Now to force the new imapd and imapd-ssl services to reload it’s config I did a restart of the courier-imap, like so:

debian:~# /etc/init.d/courier-imap restart

That’s all now the error is gone and the client could easily configure up to 50 mailbox accounts on his PC 🙂