EDIT: I forgot to mention the POP3 account has about 100K messages in it.

Do we have a google imap account in this mix, they ignore any setting you make in the user interface due to limitations in the Google IMAP implementation.

Other IMAP implementations may have their own limitations on archiving. So I think the first issue is to determine of the archive issues are only with IMAP accounts.

Matt,

Thank you for picking this up!

No, there are no Google IMAP accounts. There are two Comcast IMAP accounts and one Comcast > POP3 account. The POP3 and IMAP #1 are for the same account, i.e., POP3 (Sky) and IMAP (Sky). IMAP #2 is for a different account, i.e., IMAP (Moon).

Both IMAP accounts are set to [X] Check for new at start; [X] Check for new every xx; and [X] Allow immediate server notifications are all turned ON.

The POP3 account is set for manual fetch only. [_] Check for new at start; [_] Check for new every xx; and [_] Allow immediate server notifications are all turned OFF.

I manually fetch the email for the POP3 account to move it off the IMAP server using the 'Get New Messages' button, but occasionally I do a drag-and-drop, for instance when moving my sent mail IMAP > POP3.

Those settings|fetch|drag-drop all seem to work smoothly and as-expected. I don't think any of those settings or how I use the accounts are relevant, but that's only a guess hence the above.

I did notice yesterday that after leaving the device for several hours (during which it was powered up awaiting a backup which shuts it off when finished), when I powered it up in the morning the Local folders / Archive seemed to be intact and the POP3 / Archive appeared resolved. This leads me to wonder if it might have something to do with caching + LAN traffic + NAS speed as the NAS has HDDs (vs SSD) and is a bit slow even though the LAN is all gigabit.

If that's it archiving nearly 100K msgs is going to be a very slow process indeed. :-/

UPDATE: I only archive from the POP3, but the IMAP accounts are set to archive to the Local Folder/Archive as well. Hmmm…

In keeping with the IMAP issue you mentioned, and wondering about the CPU>LAN>NAS caching question as noted above, I am going to experiment a bit to see if any of these replicate the behavior.

1. changing the settings for the IMAP accounts and repeating the archive attempts;
2. moving the Archive onto the local CPU and repeating the archive attempts; and
3. transferring the local archive in #2 to the NAS, repeating the attempts.

I will post my findings here when finished.

RESULTS

TEST #1 | FAIL Message pane closed All Archives' subfolders permanently deleted Local Folders Message Storage set to mapped drive on NAS H:\Documents\eMail Archives\Sky POP3 Message Archives set to (*) Other: Archives on Local Folders .. Local Folders\Archives IMAP accounts archives turned OFF : [_] Keep message archives in:--- Offline > Work offline selected Select each account > Compact Folders Close & Re-launch Thunderbird

POP3 Inbox > Select single message > type 'a' > SUCCESS

Select two or more messages > type 'a' > FAIL If < 4, all fail to POP3\Archives If > 4, some msgs go to POP3\Archive; others go to Local Folders\Archives Quantity of msgs that move to NAS if > 4 appears random ________________________________________________________________

TEST #2 | FAIL — IDENTICALLY TO TEST #1 Message pane closed Archived messages moved via d&d back to originating folders All Archives' subfolders permanently deleted Local Folders Message Storage set to C:\Users\Sky\Documents\eMail Archives Thunderbird restart POP3 Message Archives set to (*) Archives Folder on: Local Folders IMAP accounts still turned OFF : [_] Keep message archives in:--- Offline > Work offline selected Select each account > Compact Folders Close & Re-launch Thunderbird

POP3 Inbox > Select single message > click 'a' > SUCCESS

Select two or more messages > click 'a' > FAIL If < 4, all fail to POP3\Archives If > 4, some msgs go to the POP3\Archive; others go to Local Folders\Archives Quantity of msgs that move to NAS if > 4 appears random ________________________________________________________________

TEST #3 | ABANDONED DUE TO FAILURE OF TEST #2

There appears to be a problem with the Archive function in this version of Thunderbird. I don't know if add-ons could be involved. I don't know how to uninstall and re-install Thunderbird without losing all of the messages & folders. I'm stuck with the only apparent alternative being to manually create the folders needed in the Archives then manually d&d my messages there.'

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EDIT: 6 Feb 2020 I probably should have mentioned this at the beginning.

Account Settings > Copies & Folders > Archive options… pops a window with graphic representations of the four options. I chose Yearly archived folders + Keep existing folder structure of archived messages.

According to the graphic if I archive something form the Inbox I should see Year/Inbox, i.e. 2010/Inbox. Instead I only get the year. If I archive something from a folder instead of the Inbox I get Year/Folder, even Year/Folder/Folder etc. (when it works partially or completely), but Year/Inbox never happens.

So it looks like this archive question is going to flounder for some time. Fwiw, this issue is still OPEN