Repeated Win Defender AV notice of Trojan:HTML/ScrInject.AW!MTB

Have you tried setting Windows Defender to bypass checking Thunderbird activities?

No - I'm still not sure these are false positives. Are you?

I am confident that Thunderbird is not creating viruses. There are millions of users and no reports. If windows defender is flagging a problem, my suggestion is to see if this relates to certain senders or other activities.

It probably isn't creating viruses, but it can certainly download emails with malware. Strange that reinstalling from the ground up has stopped the AV notifications.

Thunderbird by default does not allow remote content to be displayed, so assuming you have not changed that setting then downloading and opening email will not cause a problem.

However, if you click on links in emails without checking out what the real link is first then you may open an unsafe webpage in a browser which can force an auto download of bad file. check all links before you choose whether it's ok to click on them - hover over link to see real link in bottom Status bar.

Be wary of attachments - nefarious people will try to get you to open attachments because they are banking on you being too curious for your own good. eg: Here is your Invoice attached, so you think what invoice and promptly open the pdf. However, it;s not a real pdf, it's an exec file for something you do not want. Never allow curiosity to get the better of you. If you have not bought anything and do not expect Invoice then it isn't. Some people pretend they already have your passwords and access to your computer and they want money. The odds are on that they do not, they are scarmongering hoping to extract monies under false pretences. Do not open attachments if they are not expected or you are not sure of sender. It might use a friends email address, but does that emails sound like it's from that person. When in doubt, email or ring person to ask if it's from them.

If you think it's a bad one, delete the email and compact the folder it was in, then empty the Trash and compact trash. Compacting removes all traces of deleted and moved email from within Thunderbird profile files.

I would also advise clear your cache as well. In Thunderbird

• Settings > General
• Scroll down to 'Disk Space'
• click on 'Clear Now'

If desired you could select checkbox 'clear cache on shutdown'

I should also advise you clear cache in your browser as well.

Advice on running scans. Exit all programs Restart computer in 'Safe Mode' only so no internet connectivity and operating only basics. Run full scans OR select to scan specfic folders eg: temp

It's also possible the AV found a false positive, but better safe then sorry.

In Thunderbird

• Settings > Privacy & Security
• Security section
• Antivirus
• checkbox ' Allow antivirus clients to quarantine individual incoming messages':

By default, Thunderbird stores each mail folder locally as single mbox file. For example, there's a file called 'Inbox'. If your antivirus software detects a virus in a new incoming message, it will quarantine the entire inbox file because the new message has been saved to that file. That may not be convenient as the Inbox mbox file may have a lot of emails in it.

But with this setting enabled, Thunderbird first stores each incoming message in a temporary file in the system temp folder on your computer, allowing your antivirus software to only quarantine a single message. If the file is removed by the antivirus, then you get an error message about the file being missing. If the new message file still exists after being scanned by antivirus software, then it is moved to your Thunderbird inbox folder file.