It appears that Kapersky is intercepting the initial account connection, as TB works if not hindered initially. My suggestion is to focus on Kaspersky.

Kaspersky is probably controlling the Firewall and it is blocking Thunderbird.

Exit Thunderbird Access Firewall Thunderbird must be an allowed program in order for it to have access to internet.

Thanks for the replies.

@Toad-Hall I'd checked that Thunderbird was a Trusted application and had deleted it from Kaspersky so that it re-recognised it (and put it into Trusted) when I started Thunderbird - didn't solve the issue.

@david Yes, almost certainly Kaspersky so I'll post on their forums the same issue.

Just as a trial, disable your anti virus, restart Thunderbird. Does the initial mail check on startup occur?

@Matt I tried that. Disabled Kaspersky (paused it) started Thunderbird and the POP3 connection didn't happen.

I start the POP3 connection manually as I like to see how many emails I've got as I have lots of rules to move emails to folders so I want to know how many new emails I need to find.

I manually went to download emails from the POP3 account (Kaspersky paused before I started T'Bird) and no emails downloaded.

I then clicked again to manually download emails and they did download (Kaspersky still paused).

I tried this again with Kaspersky running (manually downloading the POP3 emails); first click nothing happened, second click to manually download and they downloaded.

So perhaps not Kaspersky after all......back to Mozilla Thunderbird :-(