Thunderbird Incoming E-Mail Problems – Blocked by Server Firewall

We recently experienced an unusual issue with Mozilla Thunderbird incoming e-mail that could explain why some users are seeing intermittent problems with a failure to receive incoming messages.

For some time Mozilla Thunderbird users have been reporting strange problems with incoming e-mail that suddenly ceases to work. We have maintained all along that the problem is probably not Thunderbird, but is more likely to be a problem associated with an ISP, a personal firewall, a server firewall, a Windows update or any number of systems that Thunderbird interacts with while retrieving e-mail. While all indications are that something other than Thunderbird is causing the majority of the problems, we did identify one potential Thunderbird issue that could be causing some of the problems that people are experiencing.

From our experience, most of the problems that we have found with either incoming or outgoing e-mail problems have been associated with ISP problems. We use Cox, AT&T and our own server. Most outgoing e-mail problems have been due to changes or problems with Cox and AT&T servers. Those tend to be intermittent and temporary. Incoming e-mail messages are a separate issue because those systems work differently. The POP e-mail system is a pretty simple system that allows an e-mail program (Thunderbird) to connect with a server, verifies a user’s account using a username and password, and then gives the e-mail system access to the user’s e-mail.

We have seen incoming e-mail temporarily go down for the Cox and AT&T accounts we use, but never for our own server. We always logically assumed that the issues were due to mail server problems at those ISPs. But recently we found that we could not access the e-mail accounts on our own server. This gave us the opportunity to troubleshoot the problem much more thoroughly.

What we found surprised us. The firewall on our server blocked our user IP address, which prevented us from accessing our e-mail. For those of you not familiar with the IP address system, an IP address is a unique numerical address that identifies each user and server on the Internet. The Internet does not operate using domain names. It runs on IP addresses. The domain name system was created as a workaround because IP addresses are very difficult to remember. When you type in a domain name or select a link on a web page, a DNS server (domain name server) looks up the IP address of the server associated with the domain name. You connect to a web site’s server using the assigned IP address. Users also have an IP address assigned to them and all communications is technically between two IP addresses.

The message in the firewall log file indicated that our IP address had been blocked after 5 unsuccessful attempts at login using an invalid password. The strange part was that all of our passwords are saved in Thunderbird and we were not previously experiencing any problems with retrieving e-mail. Taking this one step further, we did have several e-mail accounts set up in Thunderbird that automatically retrieve e-mail. I suspect that a communications problem between Thunderbird and the mail server resulted in multiple unsuccessful login attempts, which then caused the server’s firewall to block our user IP address. At this point, we do not know if the password problem was due to Thunderbird, the e-mail server or something in the communication link that caused the password to be corrupted.

The method to resolve this issue with the server firewall
You should only follow these instructions if you are currently experiencing a problem with retrieving incoming e-mail and you have verified that your POP e-mail connection settings to your ISP are correct. It may be particularly useful if your incoming e-mail was working, but now it is not.

First, disable the features for automatic retrieval of messages in Thunderbird for each of your e-mail accounts. You will find this in the Server Settings for each account. Make sure that all of the checkboxes for automatic message checking and retrieval are unchecked.

Second, you will need to have a different IP address assigned to your user account. Most ISPs assign a temporary IP address to you when you log in to your account, but that IP address may stay assigned to your account until you reboot your modem or the ISP reboots their servers. Sometimes you will have to have the ISP assign a new IP address.

If you are connected to the Internet using DSL, cable or satellite, turn off or disconnect the power to the modem, wait about 30 seconds, and then plug the power back in. If you are using a router, you may have to reboot your PC after the modem has finished rebooting. Once the modem reboots and your Internet connection comes back, refresh this web page to see if your IP address has changed. If the IP address has not changed, you may have to contact your ISP to find out how to assign a new IP address.

The final step is to check to see if you can now retrieve e-mail using Thunderbird. If you could not retrieve e-mail before using this method, but you can retrieve incoming mail now, the problem was likely related to an IP address that was blocked by the ISPs server.

Why does this happen?
To be fair, we experienced our firewall issue during a peak traffic time during the Christmas buying season. The Internet was running slow that day due to large amounts of traffic. Whenever the Internet or a server is extremely busy, strange things happen. It’s called a glitch. 🙂

ISP servers are constantly under attack. Their firewalls and systems are typically set up to detect malicious attacks and attempts to break into their servers. Some ISPs block IP addresses permanently, while others only block them temporarily. If your ISP tells you that they do not block any IP addresses, do not believe them. We recently had Cox tell us that regarding another issue, until we proved to them that their systems were blocking user access to a particular web server. All ISPs have firewalls and the primary function of a firewall is to prevent attacks. They do this by blocking IP addresses. if your IP address is blocked, you need to assign a new IP address.

One more thing to note is that since we tuned off all of automatic e-mail retrievals in Thunderbird, we have not experienced any intermittent problems with either Cox or AT&T e-mail retrieval. It may be a coincidence, but it is also possible that there is a bug in Thunderbird that does not communicate the e-mail account password correctly sometimes, and then repeatedly attempts to log into an e-mail server, which causes the server firewall to block the IP address of the user.

Let us know if this fixes your Thunderbird incoming e-mail problems.