Can you explain what you mean by "access smtp mail"? Formally, Thunderbird offers only POP and IMAP for collecting email. I don't know how you'd set it up to use SMTP without involving either of the other two protocols.

Sorry, I mean IMAP not SMTP - I think that's the send part of the system. Anyway, it's not a POP account.

Please look at your Account Settings. Under Server Settings I suspect that you have 'check for new messages at startup' selected. That's the default setting and it means that when you start Thunderbird it compares the list of messages it has already downloaded with the messages on the server, and if there are any on the server that it hasn't yet received, it downloads them. The way an IMAP account works is to try to keep the folder of messages on your computer 'in sync' with the corresponding folder on the server. So if you delete a message from your Inbox in Thunderbird it will be deleted from the Inbox on the server, and so on. Once they are downloaded by Thunderbird you should be able to access them when your computer is offline. You can check by looking at your Account Settings > Synchronisation and Storage: the option 'keep messages for this account on this computer' should be ticked. If you don't want to have your messages stored on your computer then you don't really need an email client like Thunderbird. Most email providers offer 'webmail' access where you manage your account simply by going to a web page: that way you manage the messages on the server directly and no messages are downloaded. The downloaded emails are located in your Thunderbird 'profile' which is a folder on your computer. If you are in the habit of backing up information on your machine, in case it fails, then it's a good idea to back up the profile folder regularly.

Thanks for that explanation. That's useful to know. I used to use Mulberry... I found that email client simple and straightforward and much more usable than webmail, which is rather slow and clunky. Thunderbird, likewise, is much better than the webmail interface. I just couldn't find a way to stop the downloading when I was on a pay-per-MB wifi connection, and I have thousands of emails.

In your Account Settings > Synchronisation and Storage you have options under the heading Disk Space. You can have Thunderbird download all messages 'regardless of age' or only synchronise the most recent ones (you can choose how long). If you don't need lots of old emails on your computer you may want to change that setting.