Update: The download I started while typing the question completed shortly after the question was submitted, and caught up by downloading about 250 messages, from 10/16 at 1815 through 10/17 at 1611. At that point, it was only 14 minutes behind. Retrying at 1754, no additional messages by 1850; time required seems to be erratic.

Hello

I think that you have very large IMAP accounts that you access through Imap on your phone and with Pop3 on your computer. The problem is probably that your mail provider uses a gateway for Pop3 to access the data that is primarily on an Imap server. This gateway mechanism is working fine for a server where users are deleting the messages from the server after downloading them, but it it's starting to show limits when the amount of messages on the server is huge, because the gateway has to scan a lot of messages on the Imap server to find which to present to you (the new messages). So the real solution is to drop Pop3 and to recreate your Thunderbird account as Imap. If you want Pop3 as a way to have a local copy of your emails (and backuping them yourself), I think that you should switch to a regular export of your emails from your (new) Imap Thunderbird account.

I wish it were that simple. Actually, the account started out as POP3, and the IMAP access was added for the cell phones (originally Android, migrated to iPhone). I can understand a server search being long, but why didn't they provide (as they did for IMAP) a way to start the search, say 6 months back? Why not show progress and an ETC display (there is a progress strip, but it goes to 100 percent immediately after starting!)? Or a message when complete? I am currently 40 hours behind the clock; without the slow, tedious access to pick up 3 or 4 out of several hundred messages and ignore the rest on the phone, there is no way I could keep up with billing statements. I am going through the inbox sorted by sender to move most to the Trash folder, but that is slow also. If any actual developers read this, I need a way to start "checking inbox for new messages" from, say, 6 months back, and maybe in reverse, so as to see the new ones immediately. I would be happy, once the midterm elections are over (Nov. 9th), to work with developers on adding such features. Of course, if the logic has to be changed in the servers rather than the client, that may not be possible. Currently, there are 409000 messages in the inbox (expecting a few hundred more as of today), and I'm trying to get that down as fast as possible.

13:52 moved 2210 more messages to trash, 1949 from AARP (~300/yr), emptied trash, total inbox 406790. Unable to compact folders because another operation is in progress; after typing the last clause, 132 MB saved. Those messages were also signaled to be deleted from the server, so the next attempt to get a few more might be successful after that finishes!

10:07 Thursday, after changing POP3 server name to pop.mail.yahoo.com and restarting "in flight" check for messages, started downloading message "n" of 796 (61 hours behind), then mysteriously stuck on message "6 of 796." However, cannot scroll past Tuesday 21:04 to see even the first message in this batch (Tuesday 22:08).

10:16 Friday restarted Tbird, started "get messages" ca 10:15, unread count now 275 (21 fewer) but nothing after Tuesday 21:04. "Checking inbox" disappeared ca 1023, restarted "get messages" (but not Tbird), no "wait until complete" popup, still "checking" at 11:06 ... have to leave room ... returned at 12:43, still in progress. Left house, returned at 1438, still in progress.(Friday 1257) last night finally got download restarted after deleting (via Trash, then Empty Trash) over 2000 old messages and compacting. Went from Tuesday 2104 to Thursday 1921, with one restart to get past stuck on 5th message. Restarted at bedtime, no apparent progress in morning, retried, still no progress by 1256, with Error Console window open. Contents of Error Console below:

tb.account.size_on_disk - Attempted to set the scalar to an incompatible value. 2 Successfully loaded OTR library C:\Program Files\Mozilla Thunderbird\libotr.dll OTRLib.jsm:72:13 Failed to enumerate localStorage for host about:srcdoc: [Exception... "Component is not available" nsresult: "0x80040111 (NS_ERROR_NOT_AVAILABLE)" location: "JS frame :: resource://devtools/server/actors/storage.js :: populateStoresForHost :: line 1339" data: no] storage.js:1341:17 Failed to enumerate sessionStorage for host about:srcdoc: [Exception... "Component is not available" nsresult: "0x80040111 (NS_ERROR_NOT_AVAILABLE)" location: "JS frame :: resource://devtools/server/actors/storage.js :: populateStoresForHost :: line 1339" data: no] storage.js:1341:17 Navigated to mailbox:///C:/Users/Allan%20Richardson/AppData/Roaming/Thunderbird/Profiles/rgfu6g4d.default/Mail/pop.att.yahoo.com/Inbox?number=407526 This page is in Quirks Mode. Page layout may be impacted. For Standards Mode use “”. Inbox Cookie “AWSALB” does not have a proper “SameSite” attribute value. Soon, cookies without the “SameSite” attribute or with an invalid value will be treated as “Lax”. This means that the cookie will no longer be sent in third-party contexts. If your application depends on this cookie being available in such contexts, please add the “SameSite=None“ attribute to it. To know more about the “SameSite“ attribute, read https://developer.mozilla.org/docs/Web/HTTP/Headers/Set-Cookie/SameSite je5x6d mailnews.pop3.1: Got an error name=pop3ServerBusy

Any clues?

Sorry for the delay. I think that the root cause is that: pop3ServerBusy, and it's just what I thought in my first message. There is no way to do a better POP3, it's a very limited interface and it just can't deal with huge mailboxes, IMAP was created for a reason. When you use Pop3, you really have to erase messages when you download them. Keeping an humongous amount of messages on the server is not an option.