What type of mail accounts are you talking about? Are the type of mail account's the same on all three computers?

Please check all accounts on all computers: Tools > Account Settings > Server Settings for each mail account Server type: Pop or IMAP

Report back on type of mail account.

I don't understand your question. I have the same account on all three computers so I can check it on my laptop, desktop, and at work. All the settings are identical on each computer. Nonetheless on 2 of the three devices TB keeps deleting messages from the inbox folders while on the third it does not. On one, I have 300 messages in my inbox, on the other two - zero.

re ' I have the same account on all three computers ' Yes, but what type of same account. Are they Pop mail accounts or IMAP mail accounts and have you got the same server type on all computers.

Please check and report back.

Different ways to access the information: Tools > Account Settings > Server Settings for each mail account or Menu Icon > Options > Account Settings > Server Settings for the mail account or right click on mail account name in Folder Pane and select 'Settings' then select 'Server Settings' for the mail account

it should look similar to image. At the top it will say the 'Server Type' In the example image it says : Server Type: Pop mail server.

But I need to know what Server Type you are using and I need to know if you have got the same Server Type on all computers for that mail account. see image below:

If you logon to your webmail account using a browser, are the messages all stored in various folders? Are these folders the same ones you can see in Thunderbird? If yes, then perhaps you are using IMAP mail accounts to view emails remotely. In which case you need all your mail accounts on all computers to be IMAP mail accounts. You would then need to subscribe to see those folders. You can also synchronise subscribed Imap folders to download a copy of the folder into the Thunderbird Profile folders.

If you have Pop mail accounts: Tools > Account Settings > Server Settings for the Pop mail account. you need to make sure that you select to 'Leave messages on server', then click on OK to save changes. Otherwise the messages may be downloaded to one pop mail account Inbox, but because the messages have been moved off the server, the other computers cannot download those messages. Pop mail accounts would also mean you cannot save Sent messages in a folder that can be seen by all accounts on all computers. At the moment it sounds like this is the case, but without knowing the info I've requested, it is impossible to know. But if you have pop mail accounts and want to keep using them, you must make sure they leave messages on the server.

Additional: In your situation you would be better to use IMAP mail accounts on all of your computers, then all is saved on the server which you access via any of your computers.

You cannot swap a Pop to an IMAP mail account. So, if you use Pop and want to use IMAP, then you need to make sure that the option is available to you. Logon to webmail account usng a browser and select to use IMAP forwarding. Then create the new IMAP mail account in Thunderbird. Do the same for all computers.

If you decide to use IMAP instead of Pop, move any emails in the Pop mail account into Local Folders to keep the emails before removing a Pop mail account.