when you open an attachment is it first decoded and created as a file, then the file is handed off to the helper application to view it. That file is read only. If you manage to edit it, all you are doing in modifying a file in the temp folder. it does not affect the actual attachment in the email. If you click forward then the original attachments get forwarded.

So I suppose we need to look at exactly what you are doing step by step. But I would recommend a new file name to one that is already in the email marked as removed. That does sound like a recipe for troubles.

Being more explicit . . . 1. if I just add jpg's to the email via attachment, send works fine. 2. if I add jpg's to the mail - say, several - and then view one by rt-clicking on the name in the attachment list and 'open with photo viewer', probably because I'm saying something about it in the mail text, when I later click send, I get the error message. 3. if I just remove an attachment, send works.

Basically if I do anything other than add or remove, the error occurs on send. And opening explorer, viewing from explorer works fine(doesn't disturb t'bird) but it's just extra clicks. Since the option to view is in the attachment context list, it should work correctly in all it's actions: open with viewer does work, but after closing the viewer that action alone seems to have screwed-up something in thunderbird, which now doesn't think it's connected to the server by virtue of an ID password fail, and so cannot send: re-try or change password. When this first began happening I was extremely frustrated because 1) re-try always failed, and 2) the Id/password challenge for opening t'bird worked fine. This mis-connect isn't cleared until there's a re-start. And that ought not to be happening.

have you tried editing like you say in Windows safe mode with networking. That will get your anti virus product out of the way, and I have a nasty feeling that is where you issue really lies.

When you view a file in the email a temp copy of the file is created in the temp folder. While Thunderbird should just ignore it and the open viewer. I am guessing it does not and waits for some file handle to release. Lets see if the anti virus is holding the handle open. Perhaps we can put together a bug report if enough information comes to light to reproduce it.

Editing in safe mode/ no, like explorer, just more clicks and really unrealistic.

However, you may have a handle on it, at least where the problem may be, but I think is is still within t'bird, for reasons explained.

To test, I temporarily closed the security suite (AVG Internet security), opened t'bird and I tried responding to your email ( yes, I know it's a no-reply address, but even if it 'sent' I expected it to bounce) with no text and a just 'any' picture file attachment, which listed as an attachment, I rt clicked the attachment, opened with a photo editor, reviewed, closed the editor and then 'sent' in t'bird. Previously, that was enough to get the error. This time, it went so fast I almost could not see it go. And so far, it hasn't bounced back to my in-box.

Why this action just happens when using a viewer or editor from within t'bird needs investigating. I do this sort of thing regularly from within other programs (eg, open a picture file from within a video editor to check that I've gotten the correct file before I click Ok to import it to the editor) without any negative - or any - interaction that stops the process. T'bird is the only app that reacts in this way - the action on the attachment appears to close correctly, but when the next action is send - security error, not about malware, but about not being able to connect correctly to the 'send' mail server with the right ID and password - yet both those are stored securely in t'bird, and worked when t'bird was opened to read the in-box mail.

So, clarifying further - open t'bird to check incoming mails. Opens correctly, connects to the mail server and displays the mail box. read them. - reply to one of these in-mails, with attachment, and errors as outlined.

Davik03 said

Editing in safe mode/ no, like explorer, just more clicks and really unrealistic.

Your trying to diagnose what is the problem. It is very much realistic.

However, you may have a handle on it, at least where the problem may be, but I think is is still within t'bird, for reasons explained.

I feel the issue is with AVG. While their may be a minor bug in the release of the file handles or ignoring then unreasonable delay the anti virus causes. The primary issue is with the anti virus being simply to slow and to great a drag on system resources.

You are however welcome to file a bug providing steps to reproduce here https://bugzilla.mozilla.org/ I have a feeling it will simply be noted as another anti virus caused issue and closed. But your still welcome to try.