I have tried this solution: https://support.mozilla.org/en-US/questions/1066093 Tools/Options/Advanced/General/Config. editor, double-click the preference mail.server.default.fetch_by_chunks to change its value from true to false, close Config. editor, OK, restart TB, and see if you still have problems opening attachments.

My second attempt to download an attachment seems to have worked, so I'll cross fingers.

Are your messages in the Inbox, or in another folder?

I have seen strange things with messages which include video file attachments (.avi) that are moved to a folder by a filter. Typically, the message and the video data are there, but the moved copy doesn't say it has an attachment, or reports an empty attachment.

Rebuilding the folder's index (Folder|Properties|Repair Folder) usually fixes it. Another method is to save the message as a file (you can generally drag-and-drop to the desktop) and it will create an .eml file there. Double-clicking this will open that file in Thunderbird and, hey presto, the attachment re-appears.

If you know what sort of file it was, yet another (geeky) approach is to open the message source, decode from base-64 if necessary, then copy and paste the data part into a new file, and rename it with an appropriate extension. (I do this with Notepad++.) That file will then play in the default player.

Your documents and images may not necessarily be lost.

I am still occasionally having problems saving attachments.

The messages are in my inbox. I select to save or detach one or all files. If detaching, I am prompted with message that attachment was successfully saved and do I want to continue. No matter whether I continue or not, the saved file size is zero and unusable.

I am saving to a NAS drive and the attachments are of any type (image, document, PDF). It happens only occasionally, but I hesitate to use detach at all any more.

When this happens I have noticed that the file size is unknown and when I save I get a message on the left side of the bottom bar "downloading message" .

The inbox is enabled for offline use - I have "keep messages for this account on this computer" and "synchronize all messages locally regardless of age" ticked.

I am using Thunderbird 52.9.1.

Can anyone help?

The messages are not being moved by a filter, they are lost. And I cannot recreate from the source.

Do you see any documentation anywhere suggesting that Thunderbird will work correctly on a nas device? I have been hanging around these support forums for a lot of years now and it is funny how many people install their profile onto a networks drive and than have problems. Perhaps that could be because Thunderbird is not tested on anything but a local drive. It there is a transport error in the file transfers. Thunderbird will not know unless the operating system bucks. It does not do checksums and the like that would normally be done with network file copies because there is an expectation that Thunderbird will have access to a dive that does not use the network.

Local drive have data throughput in the gigabytes per second. You might get that if you use esata ports on the NAS, but then it is not a NAS, it is a PC attached drive. But basically move your profile to an internal drive. If the issue goes away then the problem in the network/nas.

I could see Thunderbird completely saturating a standard home wifi network especially with anti virus scanning enabled., without and music streaming or anything else. Once a network is saturated anything can and does happen.

To be more accurate, I am using a profile on my local drive which is being synced to the NAS drive using Synology Cloud Station. I am sharing the same profile between 2 computers so syncing them both with the NAS. The attachment is shown as "size unknown" before I attempt to detach it, so it appears to not be pulling it from the imap server successfully?

Same attachment from the same source perhaps? unless the attachment is actually encoded in the email correctly it will report the size as unknown. It it encoded correctly? Probably not. Most Microsoft email programs do not encode correctly and most Microsoft windows developers have a cavalier attitude to mime encoding that is quite distressing in it's naivety and it's blissful ignorance.

Now moving on from my rant, Thunderbird folders are stored as a single large file, so the sync is in itself problematical. syncing a 4gb file is prone to running over itself on the network.

Back to the attachment. is it PDF file or Microsoft office? Both are known infection vectors and it would not be the first time an anti virus program completely mangled the attachment when it scanned it. But I asked about those two sources because I have only ever heard of those being the subject of anti virus manglers.

Sources are variable, file types are variable. I have received multiple emails from the same source with the same file type, one has the problem others don't. It seems to be associated with a thunderbird session. After I open Thunderbird, all emails with attachments will be affected, after I close and reopen some of them may be okay, I close and open again and they are all ok. So it could be sync status. But none of the files are more than 770 MB (global-messages-db.sqlite).