This does not appear to be isolated to a specific service provider. I’ve seen this happen on comcast.net and on gmail.com. It does appear to have to do at least with Outlook (not 2019 specifically) and Thunderbird. When a user tries to access their remote email folders via Outlook, Outlook retrieves only a portion of the folders. One recommendation in the past that I don’t adhere to is that you should just skip your favorite email client (Thunderbird or Outlook) and just use the web browser as all the folders are seen there. One issue with that is that if your folders aren’t local you can’t search them (how do you search in folders you can’t see?) nor can you manage the emails themselves.
When I was researching this my search in Google resulted in hits that there were about antivirus software getting in the way. Others said port 25 was the issue. Neither the connection issue you might encounter nor the missing folders has to do with port 25. Port 25 is for sending mail between servers not between the server and the client. Port 993 is used with IMAP and port 465 is generally used for SMTP. IMAP is for retrieving and SMTP is for sending. Port 25 has nothing whatsoever to do with this issue.
Antivirus could potentially be the cause of a connection issue when trying to send/receive/sync as some of the results/hits from the google search revealed that AV has been the cause. It isn’t the whole cause (of not being able to sync email between the server and the client).
In the situation that I encountered there were two contributing factors. So, you could have two different contributing factors also. In the case that I was working on the user was getting messages from Outlook telling him there was an issue connecting to the server and the second was that at some point he had a sync take place yet not all folders were visible in Outlook or even Thunderbird. What was much later discovered was that he’d recently changed his password — and it wasn’t corrected in Outlook. That caused the connection error message. Once that was corrected then he could connect to the server. His folders were still not sync’d even after doing a send and receive.
If your password is fine and you can talk to the server except your email folders on the remote server don’t match your client while using Outlook (or even other programs like Thunderbird), then the issue is likely in Outlook or Thunderbird centered on the subscribe folders feature.
I haven’t used Outlook in a long time and when I did I rarely used it. It must be something relatively recent in that Outlook will not show folders if you haven’t subscribed to them. You’d say “what, I never had to subscribed to my own folders”. However, Outlook thinks you must.
In order to resolve this you will need to query the folders in Outlook for that account. When you get to the screen that allows this, look for the checkbox below the list of queried folders and uncheck it. It essentially is telling Outlook to not show folders that are not specifically subscribed to. By unchecking this box you are saying that you want to see all folders regardless of the subscribed status.
For many this is a frustrating issue especially if you aren’t very familiar with Outlook. Most people using Outlook would not even think about subscribing to the folders that they created in their own email address.