gmail account? pop or imap?

Local Service provider. IMAP

CarmiDriver said

Local Service provider. IMAP

We do need to know what provider. Support is hard enough, without guessing things.

What anti virus product do you use? Does it scan your mail?

ABC Communications. It's downloading ALL the messages stored on the server every time Thunderbird is opened

Norton is the Antivirus I use. As far as I know it only checks attachments if I try to open. Can't see what this has to do with the behavior of TB, It used to only download the new messages. I'm guessing this changed with the last update. This issue continues to annoy as I cannot read any message until the download completes. I've searched though all the settings to try and find a setting to change without any result. Please help.

CarmiDriver said

Can't see what this has to do with the behavior of TB, It used to only download the new messages.

I suggest you check what Norton actually does. It's default is to scan mails. It's firewall components default is to block each new version of Thunderbird.

IF it is scanning incoming mail turn the thing off. Norton's has a nasty habit of "saving" your password into it's vault so updates to your email password just fail. It is also good at dropping the connection before the notification gets to Thunderbird of the current state of the account. It also is know to re-download mail after a mail it does not like the format of arrives. So you get some spam message that is chokes on and for ever more you get the same email downloaded over and over until you turn the email scanner off or manually delete the cause on the server. It's sonar makes adding an account impossible as well.

As far as opening attachments. The scan occurs when the attachment is saved to the temp folder when the operating system is asked to open the file. So it has nothing to do with email or the email process.

Ok. Turned off Norton email scanning. No change to the behaviour of TB. Going to do a complete restart just in case.