Thunderbird:Testing:Antivirus Related Performance Issues

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This article is not just about performance issues. Antivirus software can also cause bad or incorrect behavior, and random or consistent crashes.

The information below is anecdotal, users like you giving their personal experiences. These are not the result of organized testing. So if you have a problem that is not listed here, please help other users by adding your experience to the list. (create a wiki account, and select edit)

Related Articles


Possible symptoms, which can be consistent or random:

  • Mild to severe poor performance
  • Not able to get new mail
  • OS hangs or crashes

Including but not limited to doing the following:

  • Starting Thunderbird
  • Images not displaying
  • Opening file attachments
  • Selecting a different folder
  • Opening/displaying a message
  • Moving messages to a different folder
  • Replying to a message / Sending a message
  • Downloading or attempting to get new messages
  • Initial mailbox indexing when upgrading to 3.0.1 from a previous version
Warning signWarning: Your AV software probably worked great for ages, in previous versions of Thunderbird. But that is NO guarantee it will work well now, or in any future version of AV or Thunderbird. AV or Thunderbird updates or version changes can cause interaction problems - even if Thunderbird is the only software affected, and even if your AV version did not change. You must explicitly test by disable/enable of AV to determine the impact on Thunderbird. The quickest way to test, is to start Windows in safe mode.

First Steps

The quickest way to test in impact of other software on Thunderbird is to start Windows in safe mode.

Warning signWarning: Your AV software has probably worked great for ages. That is NO guarantee it will work well in any future version of AV or Thunderbird. When either one changes version there can be interaction problems - even if for you Thunderbird is the only software affected, and even if your AV version did not change. You must explicitly test by disable/enable of AV to determine the impact on Thunderbird. The quickest way to test, is to start Windows in safe mode.

Problem Antivirus and Anti-Malware Packages

To test whether Antivirus or Anti-Malware software is causing a problem, you must disable the software temporarily or run Windows in safe mode. Permanent solutions might require you to disable parts of the software, change to different software, or exclude Thunderbird profile folder locations from scanning. The default profile location on Windows is %appdata%\thunderbird\profiles **NOTE, if you altered default profile location or Thunderbird accounts to put mail somewhere on disk other than the standard profile location, you will need to also exclude those nonstandard locations.

Problems have been reported with:

Also, several products have caused pop download errors or slowness if incoming mail is scanned [32]

Unaffected Antivrus Packages

The following programs have generally been observed as working well with Thunderbird (but not proof that these products can't have problems):

  • AVG Antivirus - (revised) from [33] "[AVG] are no longer going to support the Thunderbird plugin because of the frequent updates of Thunderbird." And you may see problems with older versions (like 9.0.830). AVG may cause slow downloading of pop mail [34]. and AVG 9.0 may slow down sending with TB 3.1.x if set to scan outgoing messages) [35].
  • ANTIVIR 9 seems to work with Thunderbird 3 - from klint: "ANTIVIR 9 (Free version) runs fine, but ... there is no specific MAIL protection activated in the free version I have. I'm using the global protection to protect me from anything that could be launched from a mail."
  • (old assessment and perhaps no longer true ... ESET NOD32 version 8 appears to cause no issues. Although the security suite is another matter.)
  • (Others wanted, please add)

Configuration Notes and Solutions for specific Anti Virus products


AVG can not scan SSL connections and requires you to set it up as a proxy and place the mail settings in AVG and pointers to the proxy in Thunderbird. AVG offers instructions here

AVG causes intermittent Not Responding in Windows 10.[36]


McAfee has had major problems with Thunderbird over the years, causing crashes and poor performance, "Not Responding", "Unresponsive Script", and other issues. (For example [37], [38], [39], [40], [41], [42], [43], (in 2013), [44], [45], [46], [47], [48], [49], (perhaps), [50], [51], [52], [53], [54], [55], [56], bug 592303.) [57]

We recommend you do all of the following:

  • Disable/delete McAfee Anti-Spam Thunderbird Extension in Thunderbird | Tools | Add-ons | Extensions (It is safe to remove the add-on, and is absolutely not needed to have excellent virus protection.) [58],[59]
  • Disable email attachment scanning in McAfee (it individually scans all emails which slows down Thunderbird) - Uncheck "Email attachments" under Real Time Scanning > Schedule and run scans > Settings > Scan these attachments and locations or (in older versions) Real Time Scanning > Settings > Scan these attachments and locations
  • Disable scanning of Thunderbird files and folders in McAfee (by default McAfee is NOT supposed to scan them):
    1. In Windows, right+click on the McAfee icon in Windows notification area (aka system tray), or start McAfee from start menu
    2. "On-Access Scan properties ..."
    3. "All processes", "Exclusions"
    4. "Exclusions ..."
    5. Add the "Thunderbird" and "Thunderbird Profile" folders and subfolders. Folder names must finish with a backslash. NOTE, if you have customized profile or account locations for mail, then you must exclude your custom locations.
  • Please write/call McAfee support to inform them of your difficulty. McAfee cannot fix what they do not know about.

If none of the above helps and you suspect McAfee is the cause then you may need to remove McAfee using the McAfee Consumer Products Removal tool, and seek different antivirus software.

If you are uncertain that McAfee is the cause, you can test whether something started during Thunderbird or Windows startup is involved:

  • Start MS-Windows in safe mode
  • With Windows in safe mode, start in safe mode.

If the problem is gone then you must dig to find which Windows installed program or Thunderbird addon is causing the problem.


  • The above changes disable various features, but you ARE still protected from viruses by McAfee's other protection features. (quote from McAfee) "you do not need to worry, as [when] the Real Time Scan is turned on [which it is by default] it will scan all the files and attachments and will alert you if there is any infection."
  • Common question: Thunderbird has worked fine for years, with antivirus (AV) product X. So isn't my new problem in a bug Thunderbird? Answer: No, typically not. Antivirus and firewall products are often tightly linked to little details in the software it is monitoring (eg Thunderbird), and which makes them susceptible to breakage or incompatibility from Thunderbird updates. It is the vendor of product X who bears responsibility to fix the problem in such cases.

Installation issue - If thunderbird update fails and you get a 7-Zip Access Denied error when downloading a new version or updating and you are using McAfee VirusScan Enterprise + AntiSpyware Enterprise 8.8, see the advice in this thread.

F-Prot Antivirus

When TLS and SSL protocols are used, e-mail scanning either cannot scan e-mails or may block them entirely. To solve this issue, reconfigure E-mail scanning to listen to the standard unencrypted port (110) or to any unused port Fprot support article here.

Thunderbird 3.0.1 was painfully slow for me on my 2.4 GHz core 2 duo until I took the following steps:

Disabled Global Indexing:

  1. Click “Tools” menu
  2. Click “Options...” menu item
  3. Click “Advanced” tab
    4) Uncheck “Enabled Global Search and Indexer”

Add a special exclusion to Antivirus software that covers the directory in the profile where TB stores its mailbox files. In my case the software was F-Prot so the steps were:

  1. Open F-Prot Interface
  2. Click “Virus Scanners”
  3. Click “Exclusions” under the advanced heading
  4. Click “Folder...”
  5. Type “C:\Users\$Username$\AppData\Roaming\Thunderbird”
  6. Check “Also include subfolders”
  7. Click “OK”

Note that the path to thunderbird's mailbox files varies with each OS, and exception instructions are specific to your virus scanner.

My theory is this: It looks like Thunderbird 3.0.0 and above generates a huge number of file open, seek or read events causing the virus scanner to scan the mailbox files over and over again. This only seems to affect scanners that ignore file extensions and scan all files. This could be proved if someone has some IO monitoring tools and can observe how TB interacts with the mailbox files.


Slowness and errors with nsemail.HTML

The release of Avast 10.3.2223 ( We think it is that version) has resulted in issues with avasts preventing Thunderbird from functioning correctly. The workaround appears to be to create an exception in avast for the file nsemail.html in your computers temp folder. I have to say "appears to be" as Avast have not yet replied to my email. [60], [61], [62], [63], [64], [65] SUMO links [66], [67]

Hangs with "Allow anti-virus clients to quarantine individual incoming messages"

As advised by user BigDave67 in th support topic 1094117 Now what seems to have worked for me is this:

  1. I disabled Avast.
  2. Disconnected from the internet.
  3. Started Thunderbird. (It didn't lockup.)
  4. Went to Tools > Options > Security > Anti-Virus
  5. Unchecked "Allow anti-virus clients to quarantine individual incoming messages" Clicked "OK".
  6. Reconnected to the internet.
  7. Clicked on "Get Messages" (It didn't lockup.)
  8. Closed Thunderbird.
  9. Enabled Avast.
  10. Restarted Thunderbird.

Avast 2015 causes invalid mail server certificate warnings sending/receiving mail

Avast describe the cause thus "The Mail Shield in Avast 2015 uses advanced scanning method for incoming and outgoing e-mails over SSL/TLS secured connections", the reality is they perform a man in the middle hack of your connection using SSL certificates by making themselves a top level certifying authority. As Thunderbird like all quality mail client maintains it's own certificate store the Avast hack does not work. There are instructions on the avast site explaining how to make then a trusted certifying authority.

Anecdotal evidence only, no definitive testing has been done. Love if it if somebody could find the time to do some thorough AVAST and TB testing!

Avast Blocks Sending and Receiving

My problem was Avast antivirus, once I stopped the service and restarted(virus program ect) thunderbird was once again working perfectly. Its a ssl cache problem within AVAST, not thunderbird. Learned from trying to eliminate avast from list of what may be interfering with thunderbird 3.1.2

AVAST blocks images in HTML email

"So I guess my problem's solved. I just wish I knew what happened. All I did was shut down the mail protection for a day, then restarted it. I didn't change a single setting in Mozilla Mail (didn't even open the options tab). Very strange, but I'm happy now!"

"Yeah, OK, so I uninstalled Avast, restarted the computer, launched TB and voila. Images are now loading when I click "Show Remote Content". "


Examples: [68] From Thunderbird Support

  • Problem: Sending Mail Slow
  • Solution: Turn of outgoing mail scanning in BullGuard 15. Using instructions from the BullGard Support Site.

From MozillaZine Forum BullGuard Thread October 7, 2010:

  • Problem: "expanding or collapsing items (subfolders, accounts) in the Folders display often locks up TB completely and the progam goes into Not responding state. OS is Windows Vista Home Premium SP2"
  • Solution: Upgrade to Version 9: "Bullguard asked me to upgrade from Version 8 to the beta version of Verion 9 of their product. This does seem to have resolved the issue. I am now running with their Spamfilter 9.0 enabled and (fingers crossed) everything seems OK."

Norton Security

To quote a Norton support article "The email scanning feature in your Norton product cannot scan emails from the accounts that are configured for SSL. Email scanning can only scan emails from the accounts that are configured for POP3 and SMTP" Source page here This would mean that IMAP mail accounts are also not scanned, regardless of the connection security.

Norton's products have a number of exceptions, but SONAR is the one usually refered to. There are suggestions that you create a exception for Thunderbird to fix slowness. The Norton support forum has instructions here

Norton Security Deluxe is reported to cause issues with the Language reverting to English. [69]

The Global Search & Indexer

In regards to performance issues with antivirus software ... The global search & indexer normally only has impact during the initial indexing process. Performance of this feature should be acceptable after initial indexing is complete. The time needed for initial indexing normally varies from a few hours to a day, depending on the total size of all your messages.

The initial indexing process, performed upon upgrading from a prior version of Thunderbird, may be sped up by disabling on demand antivirus scanning of the Thunderbird mailbox files.

Bogus Windows System Proxies Left by Malware

Malware (usually found only on Windows systems, but could be on any system e.g. Mac or Linux) can install a bogus Internet access proxy. This proxy can slow down Thunderbird and other programs that access the Internet. Symptoms include "couldn't connect to proxy" errors, slow sending and receiving of email, and images not being displayed in HTML emails. Sometimes these can be found after a MalWareBytes scan.

Problem:Windows System Proxy blocking HTML images anecdote

  • don't think this is a MalwareBytes problem but you never know!

"FInally! Got the answer from support at AVIRA, which wouldn't update for me. (also, THunderbird wouldn't "connect to proxy").

I went to Internet Explorer -- tools-- internet options-- connections-- LAN settings. There, I unchecked "Use proxy server;..." Voila! it all works. Avira updates, Thunderbird shows images, Java updates. It all works again.

Apparently, when you do a full system scan with MalWareBytes, it puts that on for you to keep the computer from dialing out, but it doesn't uncheck it for you afterwards. How many of you use MalWareBytes? It's a good program, but check the LAN settings after scanning."

Solution: Disable Windows System Proxies

Internet Explorer | tools | internet options | connections | LAN settings and then uncheck "Use proxy server"

Solution for Spamfighter

After every upgrade you have to do the following. There seem to be two solutions from

  1. In Start Menu | Repair Spamfighter; when repair is finished restart computer and then Spamfighter Toolbar will be back!! OR
  2. more drastic solution:
    1. Close Thunderbird.
    2. Go to Start -> All Programs -> Fighters -> SPAMfighter -> Modify SPAMfighter.
    3. Click Next twice.
    4. Click on the box next to Toolbar for Mozilla Thunderbird and select Entire feature will be installed on local hard drive.
    5. Click Install and let the installation run.
    6. When you next open Thunderbird SPAMfighter will be back again

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