Maybe your email provider is doing this for you. Is it an IMAP-connected account?

What happens if you move messages to another folder?

It is an IMAP account. All other sub-folders have no limit, so if I move e-mails into sub-folders they are protected, but this is difficult to stay on top of. I also get e-mails in an old Outlook (2003) which I don't use anymore because of compatibility issues with Windows and none are deleted there, so occasionally I have to retrieve lost e-mails from there.

Is Outlook connecting to the same accounts that have this trouble in Thunderbird?

Thunderbird has settings for automatic deletion of old messages, but this is based on message age, not the quantity of messages. And quotas imposed by providers are usually based on cumulative stored message size, not message count. So this is a bit of a mystery.

Outlook is on the same account except it is a pop. (don't know what this means?). Is there any way of changing the date for auto deletion of messages?

If you mean: Tools > Account Settings > Synchronisation & Storage for imap mail account Confirm you have this selected: Select 'Don't delete any messages' Select; always keep starred messages' click on OK

Then Right click on imap Inbox and select 'Properties' click on 'Retention Policy' tab Confirm you have this selected: select: 'Use my account settings' click on OK

I would also check the server via webmail to see if you have any setting on the Inbox folder.

re I also get e-mails in an old Outlook (2003) which I don't use anymore because of compatibility issues with Windows and none are deleted there, so occasionally I have to retrieve lost e-mails from there.

This pop mail account in Outlook. Pop by default, often downloads from server and then deletes a copy off the server. Please confirm, is it occasionally downloading and deleting the copy off the server? Because this will also delete it out the Thunderbird imap mail account which is looking at the same folder.

RE: the server - Is this gmail, yahoo, comcast, BT etc ?

Thanks for your help. I did all you suggested and did not find anything that could cause the problem, but I have now set 'no messages to be deleted' in the 'Retention Policy' tab instead of 'Use my account settings'. I'll let you know how this works.

I leave Outlook closed except when I'm looking for a Thunderbird deleted message at which time it downloads all messages from the last time it was opened, but nothing seems to be deleted.

My server is 'eastlink'

Eastlink help does warn that pop mail accounts will delete off server.

Please double check Outlook: It is clear that the emails are on the server long enough for Outlook to be able to get them downloaded. Otherwise you could not rescue any 'deleted' emails in thunderbird. So this would suggest that the deletion is always occuring after downloading to Outlook. But, as pop mail accounts can auto delete off server after downloading it is worth rechecking. There are settings that can delay the delete, so that it is not always that obvious. Please check there is no setting that could delete off server after xx days for example. located this website that may help to point to where this is in Outlook.

• http://kb.site5.com/email/email-software/mail-outlook/outlook-2010-how-to-save-or-remove-emails-from-the-server/

My version of Outlook is 2003 and does not have an Account Settings option. My account settings says that e-mails are only deleted from the server when I delete then. In any event the e-mails are being deleted from Thunderbird when outlook is closed and opening Outlook doesn't appear to be deleting any e-mails. I have over 300 e-mails in my Outlook inbox. I only open Outlook when I'm looking for an old e-mail that has disappeared from Thunderbird. With the changes I have done I'm going to let the Thunderbird inbox build up to over 50 and see if the oldest ones still get deleted. If the problem persists I can still move items to sub-folders where they are protected and none get automatically deleted, although this is a pain and difficult when I'm away.