I'm not sure whether you're saying that TB doesn't try to download emails - or that it tries and fails on the first one - or that it does download occasionally.

Please clarify and provide any screenshots of error messages you are getting.

Thank you for your reply. Here is an updated summary of all of these things that began happening suddenly actually when what seems to be the day Thunderbird automatically updated 2 weeks ago:

(Please note that I verified absolutely nothing is wrong with my servers, including 1 Gmail and 2 from my private domain.)

Problem #1: Upon opening Thunderbird, my main email (among 3 I have coming into Thunderbird) recently began showing a window saying "The account (undisclosed e-mail address) is being processed. Please wait until processing is complete to get messages." After I click okay, the inbox eventually re-populates (which it should never do, and never has done this before -- it normally stays populated as soon as I open T-bird), but then Thunderbird ends up setting my Inbox view to all default settings changing the whole layout of my inbox everytime -- i.e. thread, priority, etc., where I would have to re-select and re-organize the layout everytime I open the program). Thunderbird basically seems to keep "re-assembling" that one inbox every time I open the account.

Problem #2: Then, related to problem #1 on this same account, it also begins downloading, saying "downloading 1 of 45 messages" (for example), and then it either aborts downloading the messages after the first message, or it will keep trying and will only keep re-downloading the 1st message. (Meanwhile my 2nd e-mail account with the same domain on the same exact server is functioning totally fine, and I have changed no settings on those accounts, and their server still uses the same ports it always has. I already confirmed with my host that nothing has changed there.)

Problem #3: At the same time that all of this strangeness began, Thunderbird also began to only check for new messages (for all 3 accounts) when I open it, and will not check for messages again until I close Thunderbird and then re-open it. Pushing the "get messages" button does nothing currently, when it used to do something. Meanwhile my internet is working fine. I browse fine, etc.

Thank you for any feedback!

Problem #2 - I used to get this intermittently with another mail client I used. Absolutely nothing was wrong with the client - the problem was with the email. I went online, looked at the email to make sure it was nothing I needed and then deleted it. The emails downloaded normally. This happened from time to time and I had to conclude that there was problem scripting in the email that was causing the problem. Try that and see if it works.

Problem #3 - What are your email retrieval settings in Account Settings/Server Settings. Is it still ticked to retrieve email periodically? If it is, switch it off, exit TB, re-open TB and re-tick it, choosing a slightly different time period. Works for a multiplicity of programs and is the go to method for the Windows OS when it loses a setting.