That's how IMAP works. Actions taken in Tbird are mirrored on the server. See The difference between IMAP and POP3 and FAQ Changing IMAP to POP

My account on verizon.net/aol.com is set up as a POP account, and always has been. I have had this account for many years and it has only been within the last month that I've noticed the change.

Can you post here your Thunderbird Troubleshooting information, so we can see what are your general settings: At the top right of the Thunderbird window, click the menu button (or use the regular menu at the top), then select Help: Troubleshooting Information. Click the "Copy text to clipboard" button. Do not check the box "Include account names" Then paste (Ctrl-V), the text into your response here. We only need to see the sections down to and including the "Extensions" section, so you can delete the rest.

If you use multiple accounts (on verizon) then you should also mention from that list which one is the one with issues.

Application Basics

   Name: Thunderbird
   Version: 68.12.0
   Build ID: 20200820223055

   Update Channel: release
   User Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:68.0) Gecko/20100101 Thunderbird/68.12.0
   OS: Windows_NT 10.0

   Launcher Process: Enabled
   Multiprocess Windows: 0/0
             Disabled
   Remote Processes: 0
   Enterprise Policies: Inactive
   Google Location Service Key: Missing
   Google Safebrowsing Key: Missing
   Mozilla Location Service Key: Missing
   Safe Mode: false

 Mail and News Accounts
   account1:
     INCOMING: account1, , (pop3) pop.verizon.net:995, SSL, passwordCleartext
     OUTGOING: , smtp.verizon.net:465, SSL, passwordCleartext, true

   account2:
     INCOMING: account2, , (none) Local Folders, plain, passwordCleartext

   account3:
     INCOMING: account3, , (pop3) pop.aol.com:995, SSL, OAuth2
     OUTGOING: , smtp.aol.com:465, SSL, OAuth2, true

 Crash Reports for the Last 3 Days

 Remote Processes

   Type: Count

 Extensions

     Name
     Version
     Enabled
     ID

       Amazon.com
       1.1
       true
       amazondotcom@search.mozilla.org

       Bing
       1.0
       true
       bing@search.mozilla.org

       DuckDuckGo
       1.0
       true
       ddg@search.mozilla.org

       Google
       1.0
       true
       google@search.mozilla.org

       Lightning
       68.12.0
       true
       {e2fda1a4-762b-4020-b5ad-a41df1933103}

       Twitter
       1.0
       true
       twitter@search.mozilla.org

       Wikipedia (en)
       1.0
       true
       wikipedia@search.mozilla.org

re :whenever I delete an email in Thunderbird, it is also deleted on the server I am using.

• Right click on pop account name in Folder Pane and select 'Settings'
• Select 'Server Settings' for pop account.

I presume you have selected 'Leave messages on server' - confirm this is selected. But have you selected any other option eg: 'Until I delete them' ?

If you select 'Disc Space', what option is selected eg: 'Don't delete any messages' ?

In Folder Pane, right click on Folder and select 'Properties' Select 'Retention Policy' tab What option is selected ? eg: use my accounts settings ?

You say My account on verizon.net/aol.com is set up as a POP account which sounds like you only have one account, yet the info you supplied suggests you are actually running two pop accounts. So suggest you check both account settings.

How do you access them when viewing via webmail? Are you accessing webmail via aol or yahoo etc ? Do you have emails being sent between servers so once email is moved from A to B the original is deleted?

I presume you have selected 'Leave messages on server' - confirm this is selected. But have you selected any other option eg: 'Until I delete them' ?

'The following are selected: 'check for messages at startup'; 'check for new messages every 10 minutes'; 'automatically download new messages', as well as 'leave messages on server until I delete them'.

If you select 'Disc Space', what option is selected eg: 'Don't delete any messages'?

'Yes, this is selected, as well as' always keep starred messages', which I never selected. I don't use the starred message option.'

In Folder Pane, right click on Folder and select 'Properties'.

'I don't have this option.'

The only settings I have selected have been the "leave messages on server until I delete them'. As far as I know all the others were there when I started using Thunderbird. For a time I was having problems when I tried to delete messages in Thunderbird. I kept getting a message that I have attached. Sometimes it would go away after I adjust the server setting about deleting emails after so many days. At that same time almost every time I deleted a message I would get a pop-up box asking if I wanted to compact my files. Those problems seem to have gone away since I installed the version of Thunderbird I now have.

How do you access them when viewing via webmail?

For both of the accounts I access them through mail.aol.com. I used to have two accounts at verizon.net through aol.com but one of them (peer1092) was inactivated due to no use. I had to create a new one as an aol.com account. That one also is deleting emails from the server when I delete them from Thunderbird. The new account was created on 8/8/2020.

Do you have emails being sent between servers so once email is moved from A to B the original is deleted?

There are no emails being sent between the accounts. I'm not sure what you mean about sending emails between servers.

re :'The following are selected: 'check for messages at startup'; 'check for new messages every 10 minutes'; 'automatically download new messages', as well as 'leave messages on server until I delete them'.

So you have Thunderbird Pop account set up to delete messages off pop server when you delete them in Pop account. This explains why emails are deleted off the server.

re :In Folder Pane, right click on Folder and select 'Properties'. - 'I don't have this option.' All folders that contain emails have that option. Only the mail account name folder does not have that option.

re :At that same time almost every time I deleted a message I would get a pop-up box asking if I wanted to compact my files. When you delete an email, it gets 'marked as deleted' and 'hidden'. You may see the email shown in the 'Trash' folder, but in reality, that email is still in the original folder taking up space. Why is not properly deleted ? - because you might have deleted in error and want to recover the email. It is not fully deleted until you compact the folder.

Compacting is absolutely vital for maintaining a healthy folder. It is particulary important on 'Inbox', 'Drafts' and 'Junk' folders as they see the majority of deletions and moving of emails. A lack of compacting can create a messy large file and potentially cause corruption of the file. Info on compacting: https://support.mozilla.org/en-US/kb/compacting-folders

In your last posted comment you show an error message.

Every now and again this error raises it's ugly head. It only ever occurs with people who use a Yahoo server like you. I have also seen the same silly error message once before. Obviously, their pop server does support UIDL etc because you have been allowing messages to be left on server after download for years. We have often wondered if this is just one of their server clusters that is at fault and you just happen to have used it or whether it is a cunning ploy where they are trying to get you to remove stuff still on the server to gain space. Pop accounts are expected to download and delete off server, so they usually do not have the same amount of allowable space.

In Thunderbird: I suggest you first compact the Inbox. Right click on Inbox folder and select 'Compact'. Then repeat the same for the 'Drafts' folder.

Right click on Junk folder and select 'Empty Junk' then compact the junk folder.

This should get some deleted emails properly off server.

Then logon to the webmail account via a browser : empty the webmail 'Junk'/'Spam' folder. Then delete a load of old emails from the Inbox. Finally empty the webmail Trash folder.

Doing this bit of 'housework' on the server might be sufficient to get that account able to access server again.

Here I was thinking that the instruction "leave messages on server until I delete them" meant that messages on the aol.com server would be deleted when I deleted them from the server, NOT when I deleted them in Thunderbird. How silly of me. I have now unchecked that option.

You stated that all folders that contain emails have a Properties option. I'm sorry, but I was thinking that you were still talking about the pop account name folder since I don't have a folder called "Folder". I guess if you had said instead, "in Folder Pane, right click on Inbox folder and select 'Properties'" I would have known what you were talking about. To answer your question, the only option that is checked is "Use my account settings", though others are checked or filled-in but they are grayed-out. What does this indicate?